iOS 7 support

Hi,

Thanks for your library. I added iOS 7 versions of the activity icon based on SVWebViewController by @samvermette. (I asked Sam and it's ok for him to use the icon.) Backwards compatibility with iOS 6 is ensured by checking `NSFoundationVersionNumber`.

I you like the icon, please merge and update the Cocoapods Spec repo.

Thanks,
Claus