Allows inclusion of AFNetworking in existing project.

#### The issue
If a user uses the new SDK in a project that already uses AFNetworking, the inclusion of ENAFURLConnectionOperation creates compilation errors due to duplicated constants: 

eg:
```
duplicate symbol _AFNetworkingOperationDidStartNotification in:
    /Users/gduplooy/Library/Developer/Xcode/DerivedData/TestyAPp3-glukjkaxovgstdhfzghlnwocseqp/Build/Intermediates/TestyAPp3.build/Debug-iphonesimulator/TestyAPp3.build/Objects-normal/i386/AFURLConnectionOperation.o
    /Users/gduplooy/Library/Developer/Xcode/DerivedData/TestyAPp3-glukjkaxovgstdhfzghlnwocseqp/Build/Intermediates/TestyAPp3.build/Debug-iphonesimulator/TestyAPp3.build/Objects-normal/i386/ENAFURLConnectionOperation.o

```

#### Resolution
If we are already using AFNetworking, we can define a _EVERNOTE_API_INCLUDES_AFNETWORKING_ in our prefix.pch for our project, allowing us to use the Evernote SDK alongside AFNetworking.