Add user and group builder.

I've added two new builders:

- **user** builder

```python
    folder = create(Builder('folder'))
    hugo = create(Builder('user')
                  .named('Hugo', 'Boss')
                  .with_roles('Contributor')
                  .with_roles('Editor', on=folder))
```

- **group** builder

```python
    group = create(Builder('group')
                   .titled('Administrators')
                   .with_roles('Site Administrator')
                   .with_members(hugo))
```

@phgross could you please review this? :metal: