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
and another option may be more readable
``` python
   count = len(keys)
   if count == 2:
      return False
   return True if count and key == keys[0] else not count
```
http://repl.it/XHC/5