#223 Percentage based spellheal

a variant of SpellHeal to use percentages rather than whole numbers. this would speed up dev for the vast amount of spells that use this rather than having to write math equations for them all

SpellHealPct(heal_type, percentage, current_value, caster_value, target, crit_mod, no_calc, custom_spell_name)

special changes different from SpellHeal:

• heal_type = string, "power" / "heal" no longer case sensitive (same with SpellHeal)
• percentage = float, must be > 0.0f
• current_value = bool, true (GetHP/GetPower) false (GetTotalHP/GetTotalPower)
• caster_value = bool, true (Caster HP/Power) false (Target HP/Power)