code: function generateHmac(data, key, encoding) {
      var hmac = crypto.createHmac(HMAC_SHA256_ALGORITHM, key);
      hmac.update(data);
      return hmac.digest(encoding);
  }
docs: Generate a HMAC for the given data
  @param {String} data
  @param {String} key
  @param {String} encoding
  @returns {String|Buffer} will return a Buffer if no encoding given