Major code review, cleaning the code and adding a lot of comments. Also
implements a separate store to keep entities with a reference count and
garbage collection. This closes#15.
Known issues at the moment are:
* Webplayer is no longer working, it has to be refactored.
* AlbumPage is to be implemented.
Invalidate the store at logout, emptying cached items such as artists,
albums and songs and then preventing them from quickly reappearing when
reconnecting with the same or a different account.
Closes#5.
