Only use @:generic on static targets.

Surprised this was affecting JS, hmm...
Enable live font reloading.

It turned out to be vastly simpler to NOT make Font implement
Reloadable. This is good enough to have TextSprites automagically update
when you change a font in the assets directory. Pretty awesome!
Implement radial particle emitters.

Patch from Ali, with minor tweaks. Closes #61.
Go ahead and explicitly set this to null.