Added RPG::Element class, currently it does not have extra information
added RPG::Damage class, type is used to define :hp_damage, :hp_gain,
:hp_drain ... etc, variance is the value
Added RPG::Requirement class, its a helper for defining what is needed
for sample for equipment
added RPG::Target class, one Usable or one Weapon can have multible
Targets, so they can differ in the way they act depending on user or
enemy
EquippableItem now uses Requirement for checking the can_equip?
Added Requirement::abilities and remove them from EquippableItem
Added Requirement::masteries and remove them from EquippableItem
Added Requirement::masteries and remove them from EquippableItem
Added Requirement::states and remove them from EquippableItem