Memoize elements

This patch memoizes the elements.yml file once per request. 

Previously the elements file was loaded several times per request incurring a lot of unneccesary overhead - in our app we saw ~60 file loads per request, resulting in load times up to 10 seconds. With this patch it dropped to 2 seconds.

I know that this is handled in the forthcoming 3.0 release, but that upgrade will require Rails 4 which will delay the upgrade for us. I hope you will accept this patch for the 2.6 branch so we can continue using your repo as our base.