Added Behavior Graph asset for Unity

AI/Assets/Scripts/BehaviorTree/Node.cs

@@ -0,0 +1,29 @@
+using System.Collections.Generic;
+
+namespace BehaviorTree
+{
+    public abstract class Node
+    {
+        public virtual Status Start()
+        {
+            return Status.Running;
+        }
+
+        public virtual Status End()
+        {
+            return Status.Success;
+        }
+
+        public virtual Status Update()
+        {
+            return Status.Running;
+        }
+    }
+    
+    public enum Status
+    {
+        Success,
+        Failure,
+        Running
+    }
+}