Added an option to turn user Gravatars on or off

* 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

app/helpers/application_helper.rb

@@ -561,6 +561,12 @@ module ApplicationHelper
     (@has_content && @has_content[name]) || false
   end
 
+  def gravatar_for_mail(mail, options = { })
+    if Setting.gravatar_enabled?
+      return gravatar(mail, options) rescue nil
+    end
+  end
+
   private
   
   def wiki_helper