added toggle keyboard feature

This is my first contribution to a repository on Github. I'm not sure what I am supposed to type here, so here's my motivation on forking this project and contributing to it:

Working in an enormous gallery with thousands of pictures, I felt the need to offer the users an option to navigate through the slides (each slide containing a set of thumbnails, in this case) using the keyboard arrows. Each thumbnail linked to a large version of the image with fancybox - where I also wanted to enable keyboard navigation - but then I'd have to disable/enable the bxslider keyboard navigation each time I opened the fancybox. So I did this. Hope that's useful to someone else.