Strip invalid characters from playlist names for MPD frontend

Fixes mopidy/mopidy$480 and mopidy/mopidy#474
typo in regex
Match MPD implementation and add tests
cleanup tests and code

- move tests to tests/frontends/mpd/stored_playlists_test.py

stored_playlists_test.py:
- rename test methods names to remove _config
- remove unnecessary imports, setup, teardown, variables
- sort remaining imports

mopidy/frontends/mpd/dispatcher.py:
- remove regex comment
Move tests to *protocol*/stored_playlists_test.py
Merge pull request #486 from alzeih/develop

Strip invalid characters from playlist names sent through MPD frontend

Fix #474
Fix #480