fix cocoa pods pod spec to include AFNetworking.h as it used to; fix AFNetworking.h to work on pre OS X 10.9 & pre iOS 7.0.
Merge pull request #1402 from haikusw/cocoapodspec-AFNetworking.h-fix

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