Adding the 'change' rule to Grow21. Fixes #35
Merge branch 'master' of github.com:incompl/cloak
If we play on top of an existing card, we need to remove that old card: not just destroy its object in Crafty,
but we also need to remove it from the `game.cards` array.

I'm creating a `remove` function on `Card` that accomplishes this. In addition to adding the logic for removing
cards on overlap, this also let me clean up the `removeGroupById` code.