Add collision "events" and example character besides player

You ask one if he wants ice cream, they both say yes!

This demonstrates a very simple approach for communicating collisions
between GameObjects.  I looked into using true events and delegates but
it would require a more complicated event subscription type of
implementation.

When the game starts, just follow the mario twin.  When he touches
something other than the ground, he will jump.  He will also jump when
he senses that he is starting to fall.

Questions:

Why does the animation get messed up when both MarioSprites are walking?

How do we want to handle multiple collisions in rapid succession?

Is there a better way for AI to detect that it has come to end of the
ground?  How would koopas know to change direction using our current
approach?
Merge branch 'master' of https://github.com/whtt-eric/cooper-prototypes