Subtle module and verify function for HMAC.

This splits off the constant_time_compare function from the [Fernet](https://github.com/pyca/cryptography/pull/203) pull request (Thank you @alex for getting that started) into a separate module under hazmat called subtle. (The name inspired by [Go]( http://golang.org/pkg/crypto/subtle/))

I then use this to make a secure verify function on HMAC functions which should be constant time.

- [ ] Add verify to hashes
- [ ] Possibly figure out how to test constant timeness