Start writing a plugin to handle caching

Write a simple plugin to handle caching. All the plugin currently does
is add the reference to the application cache. It does not add the cache
itself. With this commit, every page will have a reference to the cache
manifest.
Properlly create a chache file!!
Add javascript to detect if the cache is/has been updated

and prompt the user to update. This is MVP, something cleaner is needed
some cool stuff going on with caching now