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 do not use cloudfront for these images. For example, the current prod setup points to http://images.chesscomfiles.com/chess-themes/pieces/classic/136/br.png

That change was made because cloudfront was too costly for that service. In this case, it would not cost much at all, but this is a case where the setting is embedded in the code and it should be in our web app settings. That cloudfront URL might be deactivated within a few months.