Merge Image() and CoverArtImage()

Rewrite most cover art related code, to simplify and make it easier to work with.
Unify Image() and CoverArtImage() classes, use CoverArtImage() or its subclasses everywhere.

![capture du 2014-05-12 18 50 16](https://cloud.githubusercontent.com/assets/151042/2947574/e1048c5e-d9f7-11e3-95ef-a48a92a1af03.png)
![capture du 2014-05-12 18 51 35](https://cloud.githubusercontent.com/assets/151042/2947575/e106e710-d9f7-11e3-9d33-263aee5922b9.png)
![capture du 2014-05-12 19 07 44](https://cloud.githubusercontent.com/assets/151042/2947590/fd753d0c-d9f7-11e3-9fe2-d9d2ada8e354.png)