Added wiki diff.

Diff can be viewed from the page history, or directly from the project activity page for each edit.
Uses Lars Christensen's diff library.

git-svn-id: http://redmine.rubyforge.org/svn/trunk@583 e93f8b46-1217-0410-a6f0-8f06a7374b81

public/stylesheets/application.css

@@ -647,6 +647,14 @@ div.wiki img {
     margin: 6px;
 }
 
+.diff_out{
+  background: #fcc;
+}
+
+.diff_in{
+  background: #cfc;
+}
+
 #preview .preview { background: #fafbfc url(../images/draft.png); }
 
 #ajax-indicator {