Expose mobileclient stateful interface

[simon-weber]

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The webclient was stateless, whereas the mobileclient is inherently stateful. However, to ease moving from one client to another, I've currently been replicating the webclient interface with the mobileclient.

Since the backing api is stateful, though, I'd like to expose that too. If nothing else, this will speed up my testing. The resulting methods will probably look something like eg mc.get_all_playlist_changes_since(datetime).