Get list of all subscriptions.

        :type next_token: string
        :param next_token: Token returned by the previous call to
                           this method.

def get_all_subscriptions(self, next_token=None):
        """
        Get list of all subscriptions.

        :type next_token: string
        :param next_token: Token returned by the previous call to
                           this method.

        """
        params = {'ContentType' : 'JSON'}
        if next_token:
            params['NextToken'] = next_token
        response = self.make_request('ListSubscriptions', params, '/', 'GET')
        body = response.read()
        if response.status == 200:
            return json.loads(body)
        else:
            boto.log.error('%s %s' % (response.status, response.reason))
            boto.log.error('%s' % body)
            raise self.ResponseError(response.status, response.reason, body)