Include AFNetworking.h

This fixes #1949.

I'm not sure what to do with root directives

    s.public_header_files = 'AFNetworking/*.h'
    s.source_files = 'AFNetworking/AFNetworking.h'

I'm newbie in Cocoapods, so, logically they should have been doing what I'm trying to fix with this change, however, it wasn't working.