Usage: add_attack_spell(50,"nameofspell",({"path/to/spell","cast_spell",0,1}));

Taking the parameters in order.
The first sets the chance in 100 that this spell will try to be cast in
any round.
"nameofspell" is a unique name for the spell. 
"path/to/spell" i.e. "/std/spells/wizard/acid_arrow"
"cast_spell" ... usually just that, but see below.
0 this param determines whats targetted by the spell
   0 = one attacker
   1 = all attackers
   2 = everything else in the room i.e. "all"
   3 = self, i.e. heal self.

1 This param controls the level of chanting the spell does
   0 = normal, full "casting" messages
   1 = "inaudible", suppressed early warnings, as in giving a snake poison

NOTE: This works for other attacks well, i.e. to give an NPC slice

add_attack_spell(10,"slice",({"/std/commands/slice","slice",0,0}));

This should be uses in setup() in the NPC.
