Support utf-8 chars in m3u playlists

[kingosticks]

Non-ascii utf-8 chars in a .m3u playlist will cause an error similar to the following:

2015-01-02 17:22:31,232 - ERROR GStreamer encountered a general supporting library error. Debug message: /usr/local/lib/python2.7/dist-packages/mopidy/audio/playlists.py(212): _event (): /GstPlayBin2:playbin20/GstURIDecodeBin:uridecodebin4/GstDecodeBin2:decodebin21/mopidy+audio+playlists+M3uDecoder:mopidy+audio+playlists+m3udecoder1:
<type 'exceptions.UnicodeDecodeError'>: 'ascii' codec can't decode byte 0xc3 in position 31: ordinal not in range(128)

This error only occurs if you don't enable the mopidy-mpris extension (#853 for more details).
I added utf-8 characters to all the test playlists even though the other parsers use libraries rather than directly manipulating the bytestrings and are therefore unaffected.

I'm not sure if parse_urilist needs this fix too, presumably it could contain non-ascii character comments.

[jodal]

Thanks :-)

[jodal]

Cherry-picked back to the release/v0.19.x branch for inclusion in Mopidy 0.19.6.