Add OSX support for PivotalCoreKit/Foundation sub-pod.

Hello!

We wanted to use some of the helpers in the Foundation sub-pod for an OSX project. However, the podspec specifies iOS as the platform. This makes sense for the iOS specific sub-pods, but AFIAK the Foundation sub-pod can be used for any objective-c project.

We added a OSX deployment target to the Foundation subspec, and the Foundation sub-pod now seems to work with OSX.

Thanks!
Geoff & Tim