code: func (client *Client) Sign(params url.Values) (string, error) {
  	query := encodeValues(params)
  	query = strings.ToLower(query)
  	mac := hmac.New(sha1.New, []byte(client.apiSecret))
  	_, err := mac.Write([]byte(query))
  	if err != nil {
  		return "", err
  	}
  
  	signature := base64.StdEncoding.EncodeToString(mac.Sum(nil))
  	return signature, nil
  }
docs: // Sign signs the HTTP request and returns the signature as as base64 encoding