Joints & Spring system implemented

[rje...@gmail.com]

A first release of joints and springs will soon be available. There
is a new demo (number 8) showing the pin joint. the demo is a bit
crap at the moment, I'm just trying to think of something
interesting. There is also a spring force now available, in the demo
mouse over a shape, press and hold 'd' and move the mouse.

There are a few experimental performance enhancements as well:
- Shell sort instead of Insert Sort (Thanks to Zzzrrrrr). I need to
profile this.
- Early out for contact point calculation (can/should there ever be
more that 2 contact points between polygons?)

This build will be in SVN soon.