Add Support to define a valid HP range for NPC casting

These will allow us to define a valid HP range (HP of the caster) that
an NPC will cast a spell. For example NPC casting a defensive spell at

utils/sql/git/required/2018_02_01_NPC_Spells_Min_Max_HP.sql

@@ -0,0 +1,2 @@
+ALTER TABLE `npc_spells_entries` ADD `min_hp` SMALLINT(5) DEFAULT '0';
+ALTER TABLE `npc_spells_entries` ADD `max_hp` SMALLINT(5) DEFAULT '0';