Add API tests for images in tags
Fix mediafile image tests
Add image mediafile fixture
Implement extended image lists for mediafiles.

Makes the test of 80eded77b10a5fcf855114fb9120d21fcb94e089 work.
Add documentation and clean up code
Add flac fixture with images
Remove picture from full flac fixture
Implement ImageListField for FLAC
Implement ImageListField for Vorbis comments
Implement ImageListField for WMA
Implement ImageListField for MP4
Rename TagImage to Image
Make sure the png_data method is available
Test invalid image format for MP4
Add support for different coverart mime types
Add deserialize method to storage styles
Remove unused methods
Add tiff image fixture
Merge branch 'image-tags'

Conflicts:
	beets/mediafile.py