Make grenade damage more reasonable

Previous implementation of grenade damage went like this:

http://www.wolframalpha.com/input/?i=%28%288+-+x%29+%2F+8%29%5E2+*+500

New version looks like this:

http://www.wolframalpha.com/input/?i=%28-%28%28x%2F+8%29%5E+4%29+%2B+1%29+*+110

Yeah.