Added 2D shapes and layout modules

I've used OpenSCAD mostly to make parametric lasercuttable objects, see the following objects. Doing that I needed some round 2D objects, so I created a few modules for this purpose. 
I also added 2 very simple layout modules (list and grid). 
http://www.thingiverse.com/thing:28526
http://www.thingiverse.com/thing:28626