Improve lazyload behavior

[GoogleCodeExporter]

As requested on 
http://stackoverflow.com/questions/15277981/is-there-a-way-to-lazyload-images-be
fore-they-become-visible-in-the-viewport-usi.

We currently have a buffer variable in the lazyload JS that will load images 
some number of pixels before the image is visible in the viewport, but no 
option exposed to configure that buffer. Add and expose an option to allow 
users to configure this.

Original issue reported on code.google.com by j...@google.com on 12 Mar 2013 at 7:07