title: OnMouseDown() not working with unity 5.5.3f1
body: <p>Here I have a little code and I am trying to drag object with <code>OnMouseDown()</code> function.I try bellow code but is not working. I have 2D object with <code>2Dcircle collider</code>.  This 2D circle collider can't detect <code>OnMouseDown()</code>. I want to detect mouse click, if object clicked not on other screen/ camera screen.</p>
  
  <p>Here is my little code.</p>
  
  <pre><code>public  void OnMouseDown()
      {
          spring.enabled = false;
          clickedOn = true;
          Debug.Log (this.gameObject.name + " get down.");
      }
  
      public  void OnMouseUp()
      {
          spring.enabled = false;
          rgb2d.isKinematic = false;
          clickedOn = false;
          Debug.Log (this.gameObject.name + " get up.");
      }
  </code></pre>
  
  <p>This link also not working <a href="https://stackoverflow.com/questions/41391708/how-to-detect-events-on-ui-and-gameobjects-with-the-new-eventsystem-api/41392130#41392130">Link</a></p>
  
  <pre><code>public void OnPointerDown(PointerEventData eventData)
      {
          spring.enabled = false;
          clickedOn = true;
          Debug.Log (this.gameObject.name + " Was Clicked.");
      }
  
      public void OnPointerUp(PointerEventData eventData)
      {
          spring.enabled = false;
          rgb2d.isKinematic = false;
          clickedOn = false;
          Debug.Log (this.gameObject.name + " Was Clicked.");
      }
  </code></pre>