types: addition of EncryptedType

Addition of `EncryptedType` in sqlalchemy-utils types.
Maybe these methods could be named `assert_username`, `assert_access_token` etc. This way they wouldn't collide with child class methods and also child classes wouldn't need to override them (now its mandatory for child classes to override them to avoid errors). In order to avoid dependency on child classes they could take one argument (user object).

Example:
```
def assert_access_token(self, user):
    assert user.access_token == self.test_token
```