[WIP] Common Crypto Backend (OS X)

Very preliminary at this point. Lots of ugliness + it will fail all tests on travis since it assumes the availability of the commoncrypto APIs.

- [x] AES (CBC, OFB, CFB, ECB)
- [x] AES CTR (if possible)
- [ ] AES XTS (after the OpenSSL version lands)
- [x] AES GCM (private API so may not be practical)
- [x] ARC4
- [x] CAST5
- [x] 3DES (CBC, OFB, CFB)
- [x] Hashes (SHA1, SHA2 family, MD5)
- [x] HMAC
- [ ] Error Handling
- [ ] Solution for testing (and excluding on non-OS X platforms)
- [ ] General cleanup
- [ ] Fix from Apple for rdar://15589470