add cover fetching funtionality to the MagnatuneAlbum class. When image() is called, the album checks if it already has a cached image of the right size, then it creates one from the raw image, if available, and caches it if it does not, and finally, if the raw image has not been downloaded yet, it instructs a helper class to do so and makes any oberservers awrare that new metadata is available. This should most likely be generalized somewhat as it is needed by Jamendo as well