Add some const-magic to image_utils
Add support for multiple album art sizes

Save album art/thumb as found in the source and return capability of sending JPEG_SM, JPEG_MED additionally to JPEG_TN in the SOAP response. The webservice will create and cache the new thumbnail on the fly if not available.
Note: as JPEG_TN is most likely to be used, always pre-generate those thumbs.
music/album: try and use artist's album art if artist virtual folder

This works if your media is organized in a artist > albums structure
Merge branch 'multiple_sized_thumbnails' into features

Conflicts:
	albumart.c
	metadata.c
	upnphttp.c
	upnpsoap.c
Updated README
Merge branch 'multiple_sized_thumbnails' into features

Conflicts:
	README