Adding an additional size format, highlighted squares

This PR contains the updates described in #3 without any code refactoring.

todo:
 - add new size format (20px - 200px)
 - add highlighted squares
 - alias for theme (piece) and texture (board)

@marcosdsanchez 
@ajessu 
we are creating the Config object using the serializer, which means we are we are passing a config array to the serializer which creates the Config object. This is the reason for duplicate fields, because $themeIndex is populated if there is a theme key in the array and the $pieceIndex is populated if there is a piece key in the config array. This is the reason i need to have both of these fields not to break backward compatibility