`urllib.urlretrieve` was using the correct extension in most cases -- I think
when the URL ended with .jpg -- but not in every case. This was leading to files
named just "cover" and not "cover.jpg" or something else sensible. In
particular, proxied URLs don't have .jpg extensions. This generates the filename
manually so the source image always has an extension.
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 necessitated a slight refactoring in the plugin event handling mechanism.
Rather than loading all handlers up front and storing them in a module-scope
structure, we now scan for event handlers at every send(). This is probably
very slightly less efficient but allows for more flexible logic.
