Subclassing User object

By changing the static methods to class methods you can subclass the user and keep the login working. This is what I'm doing:

```
class User(ParseUser, ParseObjectMixin):
    """Parse User object."""

    def __init__(self, *args, **kwargs):
        super(User, self).__init__(*args, **kwargs)
```

Since this doesn't alter any functionality I'm hoping you could merge it.