Added "wait" into settings. This setting specifies the number of milliseconds wait before loading a image.

Problem
======
When user moves to the bottom of the page with animation, all images are loaded at the same time.

For example, push "scroll to bottom" button in https://pqrs.org/tmp/jquery_lazyload/ ,
all images (assets/red1x1_000.png - assets/red1x1_029.png) are loaded while scrolling.

Solution
========
I've added "wait" into settings.
If "wait" > 0, a image will be loaded if it shown "wait" milliseconds continuously.

Demo:
https://pqrs.org/tmp/jquery_lazyload/index.mod.html
In this page, a image is loaded if it shown 300ms or more.