Closes #2, #3: Player can lose and reload level

The player now dies and the level reloads when the player falls through
the ground.  The distance that they fall is configurable.

They can also now press `r` to reload the level.  Also, an empty game
object has been put into the scene.  This object can be used to store
information that I want to be persistent after reloading the scene, like
a death count, score, timer, or anything else I need.