Cover art support

- [x] implement database scheme
- [x] search for embedded cover
- [x] search for cover in track directory
- [x] save covers in disk cache
- [x] search for covers in disk cache
- [x] minimal ui (display covers in lower left corner)
- [x] handle existing users
- [x] LRU cache
- [ ] unit tests
> I find 'album.jpg'. quite a lot in my collection. Not sure why I guess this is the standard name that Banshee uses

Banshee stores all covers in a disk-cache, naming then as ```album-%hash%```. I also think that 'album.jpg' is quite usual name for covers and that we should include it in our list of favorite names... What do you think? maybe after ```*folder.*```?