code: function writeKeys(mnemonic_phrase, deviceTempPrivKey, devicePrevTempPrivKey, onDone){
  	var keys = {
  		mnemonic_phrase: mnemonic_phrase,
  		temp_priv_key: deviceTempPrivKey.toString('base64'),
  		prev_temp_priv_key: devicePrevTempPrivKey.toString('base64')
  	};
  	fs.writeFile(KEYS_FILENAME, JSON.stringify(keys, null, '\t'), 'utf8', function(err){
  		if (err)
  			throw Error("failed to write keys file");
  		if (onDone)
  			onDone();
  	});
  }
docs: write some config into config file
  @param {string} mnemonic_phrase - mnemonic phrase
  @param {string} deviceTempPrivKey - temp private key
  @param {string} devicePrevTempPrivKey - temp device private key
  @param {function} onDone - callback