Added basic Theme support.

Theme is set at application level (in Admin -> Settings).
For now, themes can override stylesheets only (application.css).

git-svn-id: http://redmine.rubyforge.org/svn/trunk@818 e93f8b46-1217-0410-a6f0-8f06a7374b81

lib/redmine.rb

@@ -1,6 +1,7 @@
 require 'redmine/access_control'
 require 'redmine/menu_manager'
 require 'redmine/mime_type'
+require 'redmine/themes'
 require 'redmine/plugin'
 
 begin