Spell Effect fixes and additions

Kayen: Implemented additional functionality for SE_CurrentHP utilizing base2 values. (ie limit to body type)
Kayen: Implemented SE_MitigateMeleeDamageSP (Partial Melee Rune that only is lowered if melee hits are over X amount of damage)
Kayen: Implemented SE_SpellOnAmtDmgTaken (Effect is triggered when X amount of damage is taken)
Kayen: Fix for various spell triggers/procs to now properly use their resist modifier.
Kayen: Fix to mob->ModSkillDmgTaken(skill_num, value), setting value to -1 will now properly effect all skills.