[dwm] dwm 3.3 grid mode patch

Hi all,

I wrote a small patch that maybe others can find usefull too.
It adds grid mode to DWM, so that you can easily use tools like
http://sourceforge.net/projects/clusterssh under DWM

This is my first attempt to write any useful C code, since my
daily coding consists mainly of PHP and/or Ruby, before that Perl,
and in the beginning Pascal. I've always wanted to learn C though,
but never found a good excuse to do so. Not until now, when I
really had an itch to scratch I guess :-) but more important
when I saw how clean, elegant and beautiful C can be... Reading
the DWM source code was inspiring, so thank you Anselm for this
pearl of coding.

Sincerely,
Alex

P.S. any feedback is more than welcome :-)