Add alternate comparison types to versions

- The types are:
-- Previous: The default and the previously used type
-- Subsequent: Compares against the next version
-- Current: Compares against the current version
- Allow switching between comparison types in index and diff views
-- Have links vary depending upon current comparison type

app/views/wiki_page_versions/index.html.erb

@@ -3,6 +3,8 @@
 <% content_for(:content) do %>
   <h1>Wiki Page History</h1>
 
+  <%= render "versions/types" %>
+
   <%= render "listing" %>
 
   <%= numbered_paginator(@wiki_page_versions) %>