Play defeat sound when you press L

L for “lose”

Sets up a few things:

• loads the defeat sound so we know how to play it when we implement losing
• uses code that runs a function only once when the player presses a key – now we know how to do that kind of thing
• sets up a system for prevent more than one music playing at a time
Merge branch 'gh-pages' of github.com:drexel-gmap-260-chain-gang/chained-escape