Make chain layer and put it under the motorbikes

This makes the chain look more natural; the chain is not floating on top of the player.

And refactor layers so you can specify all of their names and their order in an array
Lose the game instantly when you reach the end

Checks that you didn’t break your chain already, because maybe you broke it just before the end and still want to animate the players riding their separate ways, even past the split point.
Merge branch 'gh-pages' of github.com:drexel-gmap-260-chain-gang/chained-escape