Added OLImageResponseSerializer for AFNetworking 2.0+

I added response serializer for the image to great OLImage instances.
No `data` or `isFinished` checks are added because `AFNetworking` calls this when call is finished and data is checked for `nil` inside the `imageWithData:` code.

I also updated podspec, but it will not work until the new build is tagged. I don't know what's the correct way to update podspec when adding new files.