Generates and returns URL for redirecting to Login Page of RunKeeper,
        which is the Authorization Endpoint of Health Graph API.
        
        @param state: State string. Passed to client web application at the end
                      of the Login Process.
        @return:      URL for redirecting to RunKeeper Login Page.

def get_login_url(self, state=None):
        """Generates and returns URL for redirecting to Login Page of RunKeeper,
        which is the Authorization Endpoint of Health Graph API.
        
        @param state: State string. Passed to client web application at the end
                      of the Login Process.
        @return:      URL for redirecting to RunKeeper Login Page.  
        
        """
        payload = {'response_type': 'code',
                   'client_id': self._client_id,
                   'redirect_uri': self._redirect_uri,}
        if state is not None:
            payload['state'] = state
        return "%s?%s" % (settings.API_AUTHORIZATION_URL,
                          urllib.urlencode(payload))