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 
This comment still applies, `404` is out of place here. Mentioning exceptions only should be more than enough.
Also, reading this comment again, I think it's more understandable in the context of the change and not the context of reading the code, some time later after the change.

I'd change it to something like:

`Type("string") for int values of theme, piece, texture and board need to be to string and not int, so that they can be validated correctly. Invalid int type values are converted to 0, which are valid indexes for piece/theme and board/texture and cause unwanted results`.