Switch to registry for most of mopidy extension hooks

Fixes #601

- [x] Changelog still missing
- [x] Docs for registry should be updated to help people start using this
- [x] Test for #500 needs to be reworked.
- [x] Re-add uniqueness test for uri schemas in backends.
- [x] Update library interface based in ``mopidy-local-whoosh`` findings

I was also considering moving commands into this, but it gets complicated from a bootstrap point of view. Otherwise I considered moving the ``read/write_library`` bits into models, but that can wait. Lastly, we should probably create a at the very least a local library cookie cutter, or the first sample one to get us started.