a method to retrieve the details for all activities currently supported

        :return: dictionary of response details with activities list inside json key

         {
            'headers': { ... },
            'code': 200,
            'error': '',
            'url': 'https://api.moves-app.com/api/1.1/activities'
            'json': [
                {
                    "activity": "aerobics",
                    "geo": false,
                    "place": true,
                    "color": "bc4fff",
                    "units": "duration,calories"
                },
                ...
            ]
        }

def list_activities(self):

        ''' a method to retrieve the details for all activities currently supported

        :return: dictionary of response details with activities list inside json key

         {
            'headers': { ... },
            'code': 200,
            'error': '',
            'url': 'https://api.moves-app.com/api/1.1/activities'
            'json': [
                {
                    "activity": "aerobics",
                    "geo": false,
                    "place": true,
                    "color": "bc4fff",
                    "units": "duration,calories"
                },
                ...
            ]
        }
        '''

        title = '%s.list_activities' % self.__class__.__name__

    # construct request parameters
        url_string = '%s/activities' % self.endpoint

    # send request
        response_details = self._get_request(url_string)

        return response_details