Add completion queue/group properties to AFHTTPRequestOperationManager

The other classes (`AFHTTPSessionManager`, `AFHTTPRequestOperation`, etc.) offered the ability to set the `completionQueue` and `completionGroup` properties. I have added this to `AFHTTPRequestOperationManager`, too. Useful whenever you need/want the completion blocks resulting from `GET`, `POST`, etc. to run on a queue other that `dispatch_get_main_queue()`.