Better Explosions

[cxong]

Split from #211

http://www.gamedev.net/page/resources/_/creative/visual-arts/make-a-particle-explosion-effect-r2701

The goal is to remake the explosions in the game. Currently they are expanding balls of damaging cloud. A better explosion would have:

• Particle effects flying quickly outwards
• A flash effect #282
• Knockback
• Leave behind some flames, burnt patches