code: function multiSign (transaction, keys) {
  	var bytes = getBytes(transaction, 'multisignature');
  	var hash = crypto.createHash('sha256').update(bytes).digest();
  	var signature = naclInstance.crypto_sign_detached(hash, Buffer.from(keys.privateKey, 'hex'));
  
  	return Buffer.from(signature).toString('hex');
  }
docs: @method multiSign
  @param transaction Object
  @param keys Object
  
  @return {string}