artresizer.py instances an ArtResizer object that uses internally the PIL; ImageMagick
or a web proxy service to perform the resizing operations.
Because embedart works on input images located on filesystem it requires PIL or ImageMagick, whereas
fetchart is able to do the job with the fallback webproxy resizer.
This is the first of several commits that will modernize the beets codebase for
Python 2.6 conventions. (Compatibility with Python 2.5 is hereby abandoned.)
The interface no longer specifies the type of the image embedded in the file; it
just returns a bytestring blob. When a type must be stored, it is inferred using
the imghdr module, which shoudl reduce the potential for weird bugs when the
formats don't correspond.
