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 
is this commented out because it will be needed for the 3d pieces?

I'd delete code not needed. We have git to dig old code we'd need, if that's the case.