add POJO for TilePrefab json serialization

src/com/blarg/gdx/tilemap3d/prefabs/json/JsonTilePrefab.java

@@ -0,0 +1,8 @@
+package com.blarg.gdx.tilemap3d.prefabs.json;
+
+public class JsonTilePrefab {
+	public int width;
+	public int height;
+	public int depth;
+	public String data;
+}