AFNetworking 2 support

This pull request updates CargoBay to support AFNetworking 2.

- In the iOS 6 simulator, all tests pass
- In the iOS 7 device, all tests but 3 pass (lines 439-441 of CargoBayTests.m), but these same tests fail in the current version

This pull request also updates the AFNetworking submodule to the 2.0.0 tag.