Implemented Prev / Next Sliding (Issue #20)

New method for sliding between lighboxes (using data-lightbox-slide and
existing carousel markup). Checks whether previous or next lightbox
exists and stops the 'hidden.bs.lightbox' event from firing on
hideModal().
Merge remote-tracking branch 'origin/bootstrap3'
Merge pull request #53 from jamespegg/master

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