No navigation controls if lightbox only has one image

This hides the prev/next controls and the progress indicator when the lightbox has only one image. 

![screen shot 2014-12-09 at 14 26 09](https://cloud.githubusercontent.com/assets/2236852/5360423/42ee19fc-7fbc-11e4-925c-d4c67950b5f3.png)

It seems that `this.nextBtn`(and prev) already has the reference to the button so it may be more efficient to use that instead of making another look up.