Support AFNetworking 1.3 SSL pining

As AFNetworking 1.3.0 introduced optional SSL pining, there're two sets authentication delegate methods, which causes an incompatibility issue ofCargoBay.

When anyone using CargoBay with CocoaPods, the podspec file of AFNetworking 1.3.x will by default enable SSL pining feature by defining:

```ObjC
#define AFNETWORKING_PIN_SSL_CERTIFICATES
```

and then results in compile errors.

This should close #34.