Update Podspec to support osx

AFNetworking fully supports both iOS and OSX.
Since this is just a category on AFHTTPRequestOperationManager and
AFHTTPSessionManager the pod spec should inherit the dependencies of
the Base Spec
Fix Depreciated Warnings on requestSerializer
Merge pull request #2 from abillingsley/master

Update Podspec to support osx, removed deprecation warnings.
A big thank you for @abillingsley !!