Add default templates for HTTP 504 & 505

I use flask quite a lot for building restful web services. There often comes a time for the app to `abort(504)`, however this results in a `TemplateNotFound` exception being thrown, as flask doesn't come with a template for this HTTP error out of the box. 

This branch fixes this for HTTP 504 and 505. I scanned over http://en.wikipedia.org/wiki/List_of_HTTP_status_codes for others to add, but didn't find any that were missing and that looked like they could be useful. 

If there were some I missed which I should add to this branch, just let me know.