[ODE] Heightfield collider

[David Walters]

Hi, I've just made my first SVN checkin to add the heightfield
collision class. I hope this works for everybody! Thanks to Jason for
letting me help with this project directly, I hope to help out as much
as I can.

One note: This contribution requires you to add two new files from
ode/src - heightfield.{cpp,h} to the project/makefile/etc.

I'm not sure how to properly rebuild the project files that live in
SVN, so I'd best defer to someone else to update them to match...

Anyway, I hope this new feature will allow all of you to make some
cool outdoor environments!

Regards,
David Walters (hidden_asbestos)