Implement health modification on enemy kill; add HealOnKillModifier and update death behavior

Assets/Data/Items/Vampire's draught.asset

@@ -0,0 +1,74 @@
+%YAML 1.1
+%TAG !u! tag:unity3d.com,2011:
+--- !u!114 &11400000
+MonoBehaviour:
+  m_ObjectHideFlags: 0
+  m_CorrespondingSourceObject: {fileID: 0}
+  m_PrefabInstance: {fileID: 0}
+  m_PrefabAsset: {fileID: 0}
+  m_GameObject: {fileID: 0}
+  m_Enabled: 1
+  m_EditorHideFlags: 0
+  m_Script: {fileID: 11500000, guid: c437a5565f39495281bf1dd14dd6ba40, type: 3}
+  m_Name: Vampire's draught
+  m_EditorClassIdentifier: 
+  serializationData:
+    SerializedFormat: 2
+    SerializedBytes: 
+    ReferencedUnityObjects: []
+    SerializedBytesString: 
+    Prefab: {fileID: 0}
+    PrefabModificationsReferencedUnityObjects: []
+    PrefabModifications: []
+    SerializationNodes:
+    - Name: cures
+      Entry: 7
+      Data: 0|System.Collections.Generic.List`1[[Interfaces.IStatModifier, Assembly-CSharp]],
+        mscorlib
+    - Name: 
+      Entry: 12
+      Data: 1
+    - Name: 
+      Entry: 7
+      Data: 1|Modifiers.HealOnKillModifier, Assembly-CSharp
+    - Name: value
+      Entry: 4
+      Data: 10
+    - Name: 
+      Entry: 8
+      Data: 
+    - Name: 
+      Entry: 13
+      Data: 
+    - Name: 
+      Entry: 8
+      Data: 
+    - Name: curses
+      Entry: 7
+      Data: 2|System.Collections.Generic.List`1[[Interfaces.IStatModifier, Assembly-CSharp]],
+        mscorlib
+    - Name: 
+      Entry: 12
+      Data: 1
+    - Name: 
+      Entry: 7
+      Data: 3|Modifiers.PercentStatModifier, Assembly-CSharp
+    - Name: stat
+      Entry: 3
+      Data: 1
+    - Name: percent
+      Entry: 4
+      Data: -0.1
+    - Name: 
+      Entry: 8
+      Data: 
+    - Name: 
+      Entry: 13
+      Data: 
+    - Name: 
+      Entry: 8
+      Data: 
+  itemName: "Vampire\u2019s Draught"
+  description: 'Cure: +10 Health on Kill, Curse: MaxHealth -10%'
+  icon: {fileID: 0}
+  price: 0