[ODE] plane2D

[Geoff Carlton]

I just spent a while playing around with ODE in 2D.

It works quite nicely.  A few things I noticed, grabbing from SVN.  
Firstly, the premake folder was missing from the trunk\contrib?  Also, 
the global ERP is set at 0.2, which is a very low spongy value.  
Something more like 0.8+ would be a better default.

The old plane2d patch applies fine.  I wonder whether it should be 
applied to SVN.  It isn't perfect - it is hard coded for the Z plane, 
and has a coupled 2d motor which really could be an additional joint 
type. There is also a readme that suggests adding an extra AMotor to 
every body as well - whatever that is for it would be better 
incorporated into the joint somehow.

On the plus side, its a simple joint type that can easily be applied to 
every created body.  It makes it trivial to get 2D simulation up and 
running in a few lines of code.  That alone would make it a worthwhile 
addition.  The main thing to decide is whether it should be split into 
plane2d and motor2d, or left as one.

Geoff