Updated for AFNetworking 2.0

+ Added a dispatch group (credit #248)
+ Added a helper method for generating absolute URL string
+ Added check to prevent existing objects from being re-inserted
(credit #245)
+ Tweaked podspec to more closely match AFNetworking’s
+ Adjusted indentation
- Removed unused AFQueryByAppendingParameters method (credit #246)
- Removed excess whitespace
Directly add the HTTP operations to the queue.
workaround for [Issue #225]