updated comments, moved canvas iterative draw functions

location: scripts/canvas.js
- added drawProjectiles, drawPowerups, and yes, more comments

location: scripts/destructible.js
- removed drawDestructibles

location: scripts/main.js
- updated renderCanvas, now calls the bulk draw functions from the
CANVAS module
- added an inline bulk update for projectiles

location: script/powerup.js
- removed drawPowerUps

location: scripts/projectile.js
- removed drawProjectiles, and updateProjectiles

location: tank.js
- removed drawTanks