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
it's actually
```python
not((count == 2) or (count == 1 and self_db.key != user_dbs[0].key))
```
As AND is calculated first when parenthesis are omitted so it's not exactly the same with what you wrote.

According to [Wolfram Alpha](http://www.wolframalpha.com/input/?i=not+%28c+%3D%3D+2+or+c+%3D%3D+1+and+a+%21%3D+b%29) it could be simplified to:

```python
username_available = (count != 1 or self_db.key == user_dbs[0].key) and count != 2
```

But it will fail when `count == 0`

So maybe:

```python
if not (self_db and count):
  return not count
return count != 2 and (count != 1 or self_db.key == user_dbs[0].key)
```

Or just change the order (or use parenthesis): 
```python
username_available = not (count == 1 and self_db.key != user_dbs[0].key or count == 2)
```