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 
Erik did not authorize this library to be open sourced. With the next version, the repo will be made private. That said, the library should not be making http calls for resources like this. See my note below in the conversation.