Falling Grenades

I did the thing. The player can now make it rain grenades.

A few things worth noting:
- I created a class CPEntity to represent moving entities governed by the physics engine (the name sucks; feel free to change it if we are keeping this class)
- The values for gravity/mass are rather arbitrary; we can tweak things until everything feels nice

There were a multitude of ways this could have been done, so let me know if it's not to your liking. I did things in a way that made sense to me, but that doesn't guarantee it's the right way to do things. Have fun!