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/views/settings/_general.rhtml

@@ -46,6 +46,9 @@
 
 <p><label><%= l(:setting_feeds_limit) %></label>
 <%= text_field_tag 'settings[feeds_limit]', Setting.feeds_limit, :size => 6 %></p>
+
+<p><label><%= l(:setting_gravatar_enabled) %></label>
+<%= check_box_tag 'settings[gravatar_enabled]', 1, Setting.gravatar_enabled? %><%= hidden_field_tag 'settings[gravatar_enabled]', 0 %></p>
 </div>
 
 <%= submit_tag l(:button_save) %>