Add a "world" object that contains rigid bodies

This lets RigidBody objects not be concerned with
integration and timestepping.
Refactor variables to C# conventions
Refactor, add Shape wrapper class

The Shape class represents an object that can be both positioned
in the world and rendered. This is to work around the limitation
that most objects that fulfill this role in TgcViewer implement
two interfaces instead of having one with all these methods.
Add gravity to World and basic example