Support for APEv2 cover tags

I have added support for APEv2 cover tags. There seems to be no official specifications for these tags but they are supported by several tagging tools. The origin of the tags can be found here: http://www.hydrogenaud.io/forums/index.php?showtopic=40603, sample files here: http://www.hydrogenaud.io/forums/index.php?showtopic=40860

I have created a separate APEv2ImageStorageStyle class for the APEv2 related formats. However, this results in APEv2 tags containing vorbis comment like covers no longer being recognized. I am not sure if this is a valid use case. If you want, I can change the implementation to support both formats.
Yes, all these formats use ape tags as primary metadata format. The mutagen classes also inherit from APEv2File.