960gs implementation

[sylbal]

Hi Matt,

I've just added an 960gs implementation as I used to work with this grid system and could not get the same result refactoring my code with your grid system.

I tried to follow as much your coding/naming style and hope it makes sense to add a second grid option in the mainstream of your great library.

This was tested on a 12 grid scheme, I don't have any example to validate that it also works with 16 and 24 cols but as the only diff is in the builder there should not be any problem.

Regards, -sb