Reverse VerifyKey#verify operand ordering (fixes #61)

This uses NaCl's original operand ordering for VerifyKey#verify instead of
reversing it.

The reverse ordering was a poor decision on my part. Mea culpa. This order
is more traditional for these sorts of APIs.
Use [tag, message] ordering for all authenticators
Merge pull request #70 from cryptosphere/verify-operand-order

Reverse VerifyKey#verify operand ordering (fixes #61)