Speed improvement: binary (instead of linear) search

I had some speed issues when using original jquery_lazyload plugin when displaying lots of images.
Finally after some profiling I found that the main problem was iterating over all images in $.each.
It became especially apparent when user jumped through gallery instead of scrolling down gradually.

I replaced linear search with binary one. Now gallery is running smoothly and snappy (at least under chrome - I don't know why but under firefox it still works slow as hell)

Consider merging my changes to your plugin so that others can also benefit from it.