Passing rate limit information up to the client from the response

https://github.com/Constantine-Fry/Foursquare-API-v2/issues/75

Since this is in the FSOperation main, it should be any of the requests that use the FSOperation which, to my knowledge, is all of the responses. I explicitly check to see if the X-RateLimit-Limit and X-RateLimit-Remaining are present before doing any modifications. 

I can return the immutable copy, standby for the changes.