Improved version of wl-gravatar

[David Abrahams <dave@boostpro.com>]

I found that wl was bailing out because a gravatar image file was
corrupted, so I implemented some improvements to wl-gravatar:
https://gist.github.com/622291/2537c45de5d295ead1abf66be0c06a9a234674b0#file_wl_gravatar.el

The main differences are that:

0. If the image file turns out to be corrupt, we'll re-fetch it (once)
   in an attempt to correct the problem.

1. If we still have a corrupt file, we bail out silently so the rest
   of the message can be displayed

2. Gravatar images expire after 90 days, so people's appearance will
   eventually be updated ;-)

Regards,

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com