Better unittest integration

Attach `client` fixture to test class for easiest unittest integration:

```python
@pytest.mark.usefixtures('client_class')
class TestView:

    def login(self, email, password):
        credentials = {'email': email, 'password': password}
        return self.client.post(url_for('login'), data=credentials)

    def test_login(self):
        assert self.login('vital@example.com', 'pass').status_code == 200
```