[Issue #1410] Compile AFNetworking for iOS 6 and iOS 7

- By using __IPHONE_OS_VERSION_MAX_ALLOWED and
  __MAC_OS_X_VERSION_MAX_ALLOWED, it will allow users who have minimum
  of iOS 6 or Mac OSX 10.8 to levarage NSURLSession for users on iOS 7
  and Mac OSX 10.9