Tweak physics

[araghava]

There are a few awkward things:

• ATAT doesn't tip over when you push the top. It's actually really hard to get it to fall over. It also seems to be trembling slightly when stationary on the ground.
• The player isn't affected by gravity or other forces. A possible fix for this is to remove the resetForces() call and tweak the physics properties to let friction stop the player (like real life). We might have to change the model for the player to a cylinder or something.

[jtmzheng]

• The first is probably more to do with the meshes than with parameters
• Never seen the third before. How do I reproduce it?
• The player model will probably be a rectangular prism.

[araghava]

I really want to fix the weirdness with physics...