Add contact listener for 3D physics

src-java/play_clj/g3d_physics/ContactListener.java

@@ -0,0 +1,20 @@
+package play_clj.g3d_physics;
+
+import clojure.lang.IFn;
+
+public class ContactListener extends com.badlogic.gdx.physics.bullet.collision.ContactListener {
+	IFn started, ended;
+	
+	public ContactListener(IFn started, IFn ended) {
+		this.started = started;
+		this.ended = ended;
+	}
+	
+	public void onContactStarted(btCollisionObject a, btCollisionObject b) {
+		started.invoke(a, b);
+	}
+
+	public void onContactEnded(btCollisionObject a, btCollisionObject b) {
+		ended.invoke(a, b);
+	}
+}