Fuel bar

draws fuel bar to screen, enemy slime flies towards it, on collision (ahem, certain bounds on the game window, actually) it transfers its hp to the fuel bar.

next things to add are: 

1. making impact detect the collision (might as well use it if it's available, maybe [collision map](http://impactjs.com/documentation/class-reference/collisionmap) is what is needed?) or tightening up the "collision detection" i have
2. figure out a better way to set the velocity for the slime. The direction is correct, but the speed is slowing down as it gets closer to where it's going
3. game performance drops periodically, then picks back up — i think it's when a new enemy is instantiated. i'm guessing it has something to do with our movement calculations? might have to look at a way to either break up the load or make it run better