Add a contact
        contact_id can either be the mxit ID of a service or a Mxit user
        User authentication required with the following scope: 'contact/invite'

def add_contact(self, contact_id, scope='contact/invite'):
        """
        Add a contact
        contact_id can either be the mxit ID of a service or a Mxit user
        User authentication required with the following scope: 'contact/invite'
        """
        return _put(
            token=self.oauth.get_user_token(scope),
            uri='/user/socialgraph/contact/' + urllib.quote(contact_id)
        )