Added XmlSerializer for PlayerStats.
TODO: add save/load data for other object/components/most critical things.

NOTE: now it is possible to load player stats by calling these or similar lines:

  // Load player stats.
  var directoryPath = Path.Combine(Application.dataPath, "GameData/");
  if ( !Directory.Exists(directoryPath) )
  {
    Directory.CreateDirectory(directoryPath);
  }
  PlayerStats.loadFromFile(Path.Combine(directoryPath, "defaultPlayerStats.xml"), playerStats);