Support playing local files

[agrieser]

Tony and I discussed this and decided the best route is to use itunes to handle local file playback.

This is a significant addition to the application and will require rethinking how certain components work, such as search and playlists.

The MVP for this is a way to upload songs that are then queued. We'll need to spend some time on the whiteboard figuring out how to integrate this more thoroughly into the application.