code: function sign (transaction, keys) {
  	var hash = getHash(transaction);
  	var signature = naclInstance.crypto_sign_detached(hash, Buffer.from(keys.privateKey, 'hex'));
  
  	if (!transaction.signature) {
  		transaction.signature = Buffer.from(signature).toString('hex');
  	} else {
  		return Buffer.from(signature).toString('hex');
  	}
  }
docs: @method sign
  @param transaction Object
  @param keys Object
  
  @return {string}