Shroud changes/bugfix: only cast on weapons/armor, cast timer lowered, overwriteable.

Shroud was also causing errors with dangling timers similar to Artifice, so I've changed this spell to only cast on weapons or armor. To soften the blow a little bit, the cast time is lowered to six seconds, and the spell will now overwrite itself so you could cast on the same item over and over again if you wished. Shrouded weapons can now also be enchanted (but not identified or revealed).