Back and next operations

Buggy as hell, I need to build a state machine to control what happens
on each action.
MusicInfo as unit of play rather than string path.
Now sending messages when a song changes
Exception when switching songs fast

Exceptions can happen if songs are switched too quickly.
You can now pause and resume
Reworked playlist
Further improvements to artist field
Now getting and setting data to mongodb
Adding TrackNumber to MusicInfo
Improvements for testing
Merge branch 'MongoSources'
Default sorts
now running the hosted service again
Current song album art served by webapi
Now showing album art