Update to Lightning Sentinel

The lightning sentinel was supposed to adjust the damage it did according to the monster's original health - similar to the lightning rod. However, I seemed to have missed that bit of code out. Now fixed in cvs.

The maximum damage done by the sentinel has also been reduced, to limit how powerful the sentinel is.

I have also reduced the range of the lightning sentinel from 2000 to 1500. It was dominating kills too much.