Add RapidFire upgrade, don't create a new array of upgrades every time getRandomUpgrade is called
Remove unnecessary call to generatePath in generateWave
use a different set of sprites for missiles created by enemies (closes #17)
Make guided missiles stop tracking after a certain number of ticks (closes #1)
Make xOffsets work on the perpendicular instead (But the name stays!)
Apply x offset before applying angle changes
Add xOffset to sideshot