multi-frame GIFs

I checked a change to spambayes/ImageStripper.py that purports to handle
multi-frame images.  It selects the frame most likely to contain text as the
one with the fewest number of pixels in the background color.  That's easily
circumvented, but I'll wait until the spammers accomplish that feat.  I also
removed netpbm support.  I'll wager a beer that anyone who has netpbm either
has or can install PIL, making it the more logical choice.

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