Ouyang's updates

-Motherships are now just members of alien class
-No longer a health bar for mothership
-Collision done entirely in missile class (and no longer uses
collision.js, all defined in missile class)
-Alien deaths handled within Alien.update
-No longer a play button at start
-Temporarily disabled the mothership-seeking missiles
-EntityClass' update now updates x by vx and y by vy for generic
objects.