Fix playlist not found in for disambiguated MPD playlists

As part of the fix for https://github.com/mopidy/mopidy/issues/114 the following MPD commands don't work for playlists with disambiguated names. 

* listplaylist
* listplaylistinfo
* load

This pull request builds on the work in https://github.com/mopidy/mopidy/pull/396 with a generic MPD playlist lookup helper function and some additional tests.