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 
If I'm understanding this correctly, this is an alias for theme, right?
In that case, that variable is protected, so not accessible. Can we rename the `theme` variable to `piece`, create new getters/setters for the new variable, and make the old getter/setters deprecated, and point them to the new ones, to keep BC. No need to duplicate variables.

Not sure how this plays in with the serializer though.
If I'm understanding correctly, we'll now support setting this value by passing it either in `theme` or in `piece`? (theme being deprecated, but valid still, until the next major version).

Whatever we apply here also applies to `texture` => `board` below