Use Zend\Crypt\Hmac for CRAM-MD5

I think we should be using a built-in implementation of HMAC-MD5 for CRAM-MD5. This does add a dependency on `Zend\Crypt`.

The simple test I added I have verified passed before my change.

I think the MD5 algorithm is available on all versions of PHP which support hash_hmac (PHP >= 5.1.2), but this change may not be possible if somebody knows that to be otherwise. Some evidence: http://3v4l.org/Zm8Rp#v512 matches the hash in my added test on PHP >= 5.1.2