Separate class for local/remote data sources

[jschlyter]

I'm considering moving all of the http/file data (json/der) logic to a separate class to track file/url changes (last modified, etag, etc). Does anyone know of a good existing library tracking content?

I've written a similar class for this in another project and can reuse most of that code if we find it useful. If good enough, we might even want to move it to a separate component.

[jschlyter]

WIP: #112

[rohe]

I don't have any.