Use fn def hoisting to ease sequential code reading
Use breakout fn to clarify collision detection
Rename e vars to b 

* To suggest new "bodies" terminology rather than old "entities" terminology.
Make game.shoot into game.addBody
Can just reverse invader direction

* When had time deltas, a laggy frame at the boundary of the patrol would put  the invaders way outside the patrol zone.  In subsuquent normal ticks, the invader couldn't move back inside the patrol in one tick.  This meant their direction got reversed again.  Repeat forever.  Now there are no time deltas, so this is not a problem.
Just call load() to rewind sound before playing
Rename ship to more descriptive body.  (Used for bullets.)
Use fn def hoisting to ease sequential code reading
Replace invader directionX w speedX

* Now represents actual invader speed for slightly shorter code
Make index.html the bare essentials
Remove type="text/javascript" - is the default