Added "Aha!" support

Closes issue #17

Aha!'s [blog post](http://blog.aha.io/index.php/we-added-duo-security-two-factor-authentication-because-it-was-the-right-thing-to-do/) and [support page](http://support.aha.io/entries/36821618-Two-factor-authentication-2FA-user-set-up).