Add powerups.

Add five kinds of powerups:
* Health drops, that add to the user's health.
* Rocket drops, that give the use a rocket.
* Speed boost, that increases the user's movement speed.
* Damage boost, that increases the damage done by the user's bullets.
* Fire rate boost, that increases the rate at which the user can fire bullets.

The code is kind of scattered all over the place, so here's a break down
of the changes:
* IngameCM.js tells the powerups to move and draw themselves.
* EnemyWaveInfo.js randomly creates new powerups when enemies die.
* Player.js checks to see if it has collided with powerups, and updates it
  stats to reflect the effects of the powerup (via the updateStats
  method, which is called when the powerups effects begin and end).
* Round.js makes sure that powerups that are currently in effect when the
  level ends cease their effects, and destroys powerups that the user
  hasn't picked up when the level ends.
* Powerup.js randomly chooses what kind of powerup it will be, and handles
  the timing of when the powerup's effects stop.