Adding podspec file.

This adds a basic minimal `podspec` file to facilitate inclusion of this ARC-compatible SFHFKeychainUtils fork via Cocoapods (as `SFHFKeychainUtils-ARC`).  It would also be great if the `podspec` file were submitted to the main Cocoapods repo, as described at http://guides.cocoapods.org/making/getting-setup-with-trunk