* Option can be found in Administration > General, called
  "Use Gravatar user icons"
* Defaulting Gravatars to off
* Added a helper gravatar_for_mail to check the setting before rendering
  the Gravatar.

  #1776


git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@1972 e93f8b46-1217-0410-a6f0-8f06a7374b81