Added ability to use Buffs / Restorative items

Functionality to use buffs and restorative items have been added as
function calls inside the heal_up method.  Buffs are used when the
heal_up function finishes (since we don't want to waste buff time
waiting to heal).

Using restorative items happens alongside where vigor would be used but
ONLY when there isn't enough mana to cast it and ONLY in combat when the
character's health is less than 50% of HEALTH_TO_HEAL