World artifact power descriptions

Updated on use descriptions for several World Artifacts and added some log messages.

Changed Umbraphage's damage calculation to actually care about it's charge level (slight nerf).

Fixed a bug with Butcher that could cause its rampage not to activate on damage. Now benefits from Charm Mastery.

Stormlash's AOE effect is correctly centered on the target, rather than the user.

DamageType.LITE now reduces the unlit value of grids with darkness if it can't remove them. (Fixes a possible abusable bug with Umbraphage.)

Updated the tooltip for the Invisibility effect.