[xmonad] StackTile, a new tiling algoritm.

I recentley started to use xmonad again after a while with DWM. With DWM i used a patch that gave me a tilingalgoritm that stacked the windows like in the dishes tiling, but with the ability to resize the master pane. I did'nt find such a path for xmonad so I wrote my own. I find this kind of tiling superior on small screens. I also attached the darcs patch. /Rickard