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 
Given that these last two parameters (urls) need to be in a specific format, so that certain places can be replaced, I'd add doc block to this class with examples.

`ex. /boards/__BOARD_TEXTURE__/__SIZE__.png`