Add request post strategy

Added a strategy for POST parameters as cache keys. Sometimes you want to cache posts (when  get would have been nicer but request length limits force you to go  with post).

This is a known issue: very large get requests can break a ton of stuff. Sometimes although it is not RESTful you have to issue those as GET request (you do not wan to go through the hoops of PRG and create a new resource just to do a search with large parameters).

This might have as an undesirable side effect issuing 304 against a post  if the user uses "cache_or_304" method. But anyway, users should not use this method for  post.