Be more specific about when you can trust authentication on GCM
Merge pull request #281 from alex/be-more-specific

Be more specific about when you can trust authentication on GCM
Prototype CommonCrypto (OS X, iOS) support.

* Requires OS X 10.7+ or (presumably) iOS 6+
* Supports AES, 3DES, Blowfish, CAST5, and ARC4
* Supports SHA-1, SHA-2 family, MD5
* Supports HMAC
* No way to check if the binding is available on your platform except
  try/except on the import