Build a framework for a maintainable combat plugin

* Uses a hierarchy of WeaponClasses -> Weapons for performing attacks,
  with WeaponClass having a set of styles and associated Attacks for
  those styles. Weapons and their classes are built with an easy to use
  and clean DSL.

* Adds a BonusContainer mixin, so that Equipment, Weapons, and
  WeaponClasses can all have their own set of bonuses which apply to the
  player.

* Allows attacks to be queued to the Mobs CombatState instance from
  external code, allowing e.g., NPCs or auto-cast to queue attacks to be
  executed.

data/plugins/combat/attack_requirements.rb

@@ -0,0 +1,56 @@
+java_import 'org.apollo.cache.def.ItemDefinition'
+
+class AttackRequirementException < Exception
+  attr_reader :message
+
+  def initialize(message)
+    @message = message
+  end
+end
+
+class AttackRequirement
+  def validate!(player)
+    throw RuntimeError.new('validate! not implemented')
+  end
+
+  def apply(player)
+    throw RuntimeError.new('apply not implemented')
+  end
+end
+
+class SpecialEnergyRequirement < AttackRequirement
+  def initialize(amount)
+    @amount = amount
+  end
+
+  def validate!(player)
+    throw AttackRequirementException.new('Not enough special attack energy.') unless player.special_energy >= @amount
+  end
+
+  def apply(player)
+    player.special_energy = player.special_energy - @amount
+  end
+end
+
+class ItemRequirement < AttackRequirement
+  def initialize(item, amount)
+    @item   = item
+    @amount = amount
+  end
+
+  def validate!(player)
+    throw AttackRequirementException.new(item_missing_message) unless player.inventory.get_amount(@item) >= @amount
+  end
+
+  def apply(player)
+    player.inventory.remove(@item, @amount)
+  end
+
+  private
+
+  def item_missing_message
+    definition = ItemDefinition.lookup(@item)
+
+    "You don't have enough #{lookup_item(@item).name}s"
+  end
+end