Default implementation of AccountLookup

[JoshOrndorff]

To use nimbus in the runtime, you need some implementation of the AccountLookup trait. A simple reusable implementation called the AuthorMapping exists in moonbeam: https://github.com/purestake/moonbeam/tree/master/pallets/author-mapping

This implementation is sufficiently re-usable that it should live here. In fact, it is so reusable that I wonder if it should just be part of the author-inherent (soon to be renamed nimbus system) pallet.