Subscribe to a Topic.

        :type topic: string
        :param topic: The name of the new topic.

        :type protocol: string
        :param protocol: The protocol used to communicate with
                         the subscriber.  Current choices are:
                         email|email-json|http|https|sqs

        :type endpoint: string
        :param endpoint: The location of the endpoint for
                         the subscriber.
                         * For email, this would be a valid email address
                         * For email-json, this would be a valid email address
                         * For http, this would be a URL beginning with http
                         * For https, this would be a URL beginning with https
                         * For sqs, this would be the ARN of an SQS Queue

def subscribe(self, topic, protocol, endpoint):
        """
        Subscribe to a Topic.

        :type topic: string
        :param topic: The name of the new topic.

        :type protocol: string
        :param protocol: The protocol used to communicate with
                         the subscriber.  Current choices are:
                         email|email-json|http|https|sqs

        :type endpoint: string
        :param endpoint: The location of the endpoint for
                         the subscriber.
                         * For email, this would be a valid email address
                         * For email-json, this would be a valid email address
                         * For http, this would be a URL beginning with http
                         * For https, this would be a URL beginning with https
                         * For sqs, this would be the ARN of an SQS Queue

        """
        params = {'ContentType' : 'JSON',
                  'TopicArn' : topic,
                  'Protocol' : protocol,
                  'Endpoint' : endpoint}
        response = self.make_request('Subscribe', 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)