Use Mystery Man as default avatar instead of logo

Currently if a user hasn't set up a gravatar, we
show the gravatar logo, which draws attention away
from existing avatars.

Instead, we could show just a silhouetted outline.

See http://secure.gravatar.com/site/implement/images/
Merge pull request #60 from nb/prettier-default-gravatars

Use Mystery Man as default avatar instead of logo