Checking if the email already exists

It checks only against verified users. Any feedback is welcome..

P.S. Since there is no parent in the `User` model there might be issues with [eventual consistency](https://developers.google.com/appengine/docs/python/datastore/structuring_for_strong_consistency) thingy but in real life situations if there is ever going to be a conflict with that.. it could be solved with a support ticket :D
in the ideal world it shouldn't... but it might happen that there are two users with the same username or email.. even if just editing through the gae admin console or just due to eventually consistency thingy