Allow if option for caches_page

This is a patch to allow an if option to be passed to caches_page, much as you can for session management. e.g.

caches_page :index, :if => Proc.new { |c| !c.request.format.json? }

I've included a test, and split the call to caches_page in the test controller into two, so that caches_page is called with and without an if option.

I've also moved the test for whether an action should be cached out of the block passed to after_filter, and instead pass the actions we want caching to after_filter via the only option.