Support for AFNetworking 2.0

Here's a summary of the changes:
* `OVCModelResponseSerializer` serializes JSON responses into model objects.
* `OVCSocialRequestSerializer` serializes requests into `ACAccount` authenticated requests.
* `OVCClient` now inherits from `AFHTTPRequestOperationManager`. The interface remains almost the same.
* `OVCRequestOperation` has been deprecated.
* `OVCSocialClient` has been deprecated. Use the new `+clientWithBaseURL:account:` method in `OVCClient` instead.

closes #16