Implemented Prev / Next Sliding (Issue #20 & #47)

Put in a new method for sliding between lightboxes. Previous and Next buttons can be implemented like the Bootstrap 3 carousel buttons:

	<a class="left carousel-control lightbox-control" href="#" data-lightbox-slide="prev">
		<span class="glyphicon glyphicon-chevron-left"></span>
	</a>
	<a class="right carousel-control lightbox-control" href="#" data-lightbox-slide="next">
		<span class="glyphicon glyphicon-chevron-right"></span>
	</a>	

This should go immediately after the `img` tag. The important bits to note above is the `lightbox-control` class and `data-lightbox-slide`.

I've had to change the `hide()` and `hideModal()` functions slightly so that it doesn't trigger `hidden.bs.lightbox` (otherwise you get two scrollbars on the screen).