pushy.go
func (p *Pushy) SubscribeToTopic(deviceID string, topics ...string) (*SimpleSuccess, *Error, error) {
	url := fmt.Sprintf("%s/devices/subscribe?api_key=%s", p.APIEndpoint, p.APIToken)
	request := DeviceSubscriptionRequest{
		Token:  deviceID,
		Topics: topics,
	}
	var success *SimpleSuccess
	var pushyErr *Error
	err := post(p.httpClient, url, request, &success, &pushyErr)
	return success, pushyErr, err
}
func (p *Pushy) SubscribeToTopic(deviceID string, topics ...string) (*SimpleSuccess, *Error, error) {
	url := fmt.Sprintf("%s/devices/subscribe?api_key=%s", p.APIEndpoint, p.APIToken)
	request := DeviceSubscriptionRequest{
		Token:  deviceID,
		Topics: topics,
	}
	var success *SimpleSuccess
	var pushyErr *Error
	err := post(p.httpClient, url, request, &success, &pushyErr)
	return success, pushyErr, err
}
// SubscribeToTopic subscribes a particular device to topics (when you want to do from backend)