[iOS] NSURLConnection replacement with NSURLSession

Migrate NSURLConnection instances to NSURLSession #344 (breaking change for pre iOS 7)

Nothing too fancy, but should enable more efficient/advanced use of networking if required later.
No, I'm asking if we should delete " dispatch_async(dispatch_get_main_queue(), ^{ }" from this part of the if block.  I believe it was there as NSURLConnection would do all callbacks on the same thread it was made on. With NSURLSession that no longer matters (since the call back will be on the passed in queue).