Refactors backpressure

This version includes refactoring for the backpressure operators:

• Breaking Change: debounce is now split into debounce and debounceTime.
• Breaking Change: sample is now split into sample and sampleTime.
• Breaking Change: throttle is now split into throttle and throttleTime.

Fixes and new Features

• Breaking Change: BehaviorSubject now has a separate factory constructor seeded()
  This allows you to seed this Subject with a null value.
• Breaking Change: BehaviorSubject will now emit an Error, if the last event was also an Error.
  Before, when an Error occurred before a listen, the subscriber would not be notified of that Error.
  To refactor, simply change all occurences of BehaviorSubject(seedValue: value) to BehaviorSubject.seeded(value)
• Added the groupBy operator
• Bugix: doOnCancel: will now await the cancel result, if it is a Future.
• Removed: bufferWithCount, windowWithCount, tween
  Please use bufferCount and windowCount, tween is removed, because it never was an official Rx spec.
• Updated Flutter example to work with the latest Flutter stable.

fixes and new features

• Breaking Change: bufferCount had buggy behavior when using startBufferEvery (was skip previously)
  If you were relying on bufferCount with skip greater than 1 before, then you may have noticed
  erroneous behavior.
  • Breaking Change: repeat is no longer an operator which simply repeats the last emitted event n-times,
    instead this is now an Observable factory method which takes a StreamFactory and a count parameter.
    This will cause each repeat cycle to create a fresh Observable sequence.
  • mapTo is a new operator, which works just like map, but instead of taking a mapper Function, it takes
    a single value where each event is mapped to.
  • Bugfix: switchIfEmpty now correctly calls onDone
  • combineLatest and zip can now take any amount of Streams:
    • combineLatest2-9 & zip2-9 functionality unchanged, but now use a new path for construction.
    • adds combineLatest and zipLatest which allows you to pass through an Iterable<Stream> and a combiner that takes a List when any source emits a change.
    • adds combineLatestList / zipList which allows you to take in an Iterable<Stream> and emit a Observable<List> with the values. Just a convenience factory if all you want is the list!
    • Constructors are provided by the Stream implementation directly
  • Bugfix: Subjects that are transformed will now correctly return a new Observable where isBroadcast is true (was false before)
  • Remove deprecated operators which were replaced long ago: bufferWithCount, windowWithCount, amb, flatMapLatest

Async Bugfixes

• Fix error with FlatMapLatest where it was not properly cancelled in some scenarios
• Remove additional async methods on Stream handlers unless they're shown to solve a problem

Proper error handling, remove `call`

• Remove call operator / StreamTransformer entirely. Please use do / DoStreamTransformer.
• Important bug fix: Errors thrown within any Stream or Operator will now be properly sent to the StreamSubscription.
• Improve overall handling of errors throughout the library to ensure they're handled correctly

doOnX and distinctUnique

• Added doOn* operators in place of call.
• Added DoStreamTransformer as a replacement for CallStreamTransformer
• Deprecated call and CallStreamTransformer. Please use the appropriate doOnX operator / transformer.
• Added distinctUnique. Emits items if they've never been emitted before. Same as to Rx#distinct.

Remove GroupBy

Breaking Api Change

• Observable.groupBy has been removed in order to be compatible with the next version of the Stream class in Dart 1.24.0, which includes this method

Small documentation fixes

• Remove ConcatMap link
• Change BehaviorSubject link

Closing in on 1.0-beta

• Api Changes
  • Observable
    • Remove all deprecated methods, including:
      • observable factory -- replaced by the constructor new Observable()
      • combineLatest -- replaced by Strong-Mode versions combineLatest2 - combineLatest9
      • zip -- replaced by Strong-Mode versions zip2 - zip9
    • Support asObservable conversion from Future-returning methods. e.g. new Observable.fromIterable([1, 2]).first.asObservable()
    • Max and Min now return a Future of the Max or Min value, rather than a stream of increasing or decreasing values.
    • Add cast operator
    • Remove ConcatMapStreamTransformer -- functionality is already supported by asyncExpand. Keep the concatMap method as an alias.
  • Subjects
    • BehaviourSubject has been renamed to BehaviorSubject
    • The subjects have been rewritten and include far more testing
    • In keeping with the Rx idea of Subjects, they are broadcast-only
• Documentation -- extensive documentation has been added to the library with explanations and examples for each Future, Stream & Transformer.
  • Docs detailing the differences between RxDart and raw Observables.