Fix for AFNetworking.h left out of cocoapod for AFNetworking. Also Needed conditionalization of Session includes.

AFNetworking.h was left out of the cocoapodspec.  AFNetworking.h was always including the Session based includes but they are only valid for OS X 10.9+ and iOS 7.0+ so added conditionalized imports for them.

(note related issue:  https://github.com/AFNetworking/AFNetworking/issues/1378)