backend: Add library.browse()
core: Add library.browse()
mpd: Include dirs and files in lsinfo response
core: Use library name instead of URI scheme in browse()
models: Add Ref.type constants
backend: Rename library.name to library.root_directory_name
core: Check if library is browsable at startup
local: Change library.{name => root_directory_name}
Merge branch 'develop' into feature/library-browse

Conflicts:
	mopidy/backends/local/json/library.py
	mopidy/core/actor.py
	tests/backends/local/library_test.py
models: Add type specific constructors to Ref
models: Use new Ref constructors
mpd: Use Ref type constants
Merge pull request #626 from jodal/feature/library-browse

Library browsing support in backend and core API and MPD lsinfo command