Remove broken caching

Nothing is ever evicted from the album, artist and track caches. It
also appears to cause albums to return incorrect metadata in some
circumstances.
Use NSCache to cache images

This may or may not be useful in the long run but it's better than a
dictionary.
Remove notifications from SPTrack

Registering every track for notifications is unnecessary. It can be
better handled elsewhere, especially now that caching has been removed.
Update project file for Xcode 5