Fix two bugs with CommonCrypto GCM that can result in invalid output.

Bug #1: Call to AAD but no call to update. Get null tag bytes.
Bug #2: Call to update without call to AAD. Get null ciphertext bytes.

Fixes #1329
byte strings are byte strings
moved GCM tests to be run against all backends, added radar bug numbers
resolve GCM tag issue with AAD only on OpenSSL 1.0.1 in Ubuntu 12.04
Merge pull request #1330 from reaperhulk/fix-commoncrypto-gcm

Fix two bugs with CommonCrypto GCM that can result in invalid output.