Encode uri path parts as UTF8 to avoid issues with Unicode playlist names

mopidy-vkontakte crashes on start when the user has playlists with a Unicode (e.g. Cyrillic) names.

Example stacktrace: http://pastebin.com/ded0mNrb

The reason is that urllib does not support Unicode symbols. See http://stackoverflow.com/questions/5557849/is-there-a-unicode-ready-substitute-i-can-use-for-urllib-quote-and-urllib-unquot for more context.