New spell: ice nova, Faren 5, replaces shatter

This request introduces a new spell in level 5 Faren, ice nova, which is a small (6 radius) area of effect cold damage spell which does medium damage (think 10-20) and is instant cast. This is only the "first draft" upload so to speak; the damage mechanism still needs redoing or refining as most of it was borrowed from earthquake. I've removed the shatter spell from the Faren priestess and replaced it with ice nova however I haven't disabled the spell in any other way as of yet. There is also graphics for the spell icon/description which aren't being uploaded until I'm sure where to put them.

Shatter has been removed from the monsters that cast it and replaced with ice nova. Shatter wands still exist however I don't think they're very common if they are in the game at all.

Also included is a new function in room.kod, AreaEffectTargets, which returns valid targets within a given radius.