mpd: Implement "listall" command
mpd: Implement "listallinfo" command (fixes #145)
docs: Update changelog
docs: Remove browsing from list of MPD limitations
audio: Ensure tests can be run on their own
audio: Nest tags in scan data return value
audio: Change audio data convert to operate on taglists
models: Make .copy(foo=None) null out field.
audio: Update how translator test data is built.
audio: Add tests for all fields that can be converted
model: Update to handle None in sets.

Also adds some missing tests for composers and performers.
audio: Cleanup translator code.

This code was trying to be to smart for it's own good. This commit greatly
reduces the magic and leaves us with much more straight forward code.
docs: Add config to frontend and backend examples.
Merge pull request #643 from jodal/feature/mpd-listall

Add "listall" and "listallinfo" MPD commands
Bump version to 0.18.0a1 for easier testing of updated extensions
audio: Review fixes
Merge pull request #644 from adamcik/feature/scanner-tag-handling

Cleanup taglist handling and translation to tracks.
docs: Fix syntax error
local: Replace invalid UTF-8 data from library
backend/core: Switch to root_directory instead of name