Support parallax scrolling

[daybrush]

Description

Support parallax scrolling.
If item has parallax class name and image is longer than the area surronding this, apply parallax scrolling.

Steps to check or reproduce

Use only option parallax.
Parallax's options have selector, strength, ...
selector option indicates the image to parallax scroll.
strength option indicates parallax scroll speed.

If item has not the image indicated selector, this item does not apply parallax scrolling.

var ig = new eg.InfiniteGrid(container, {
	parallax: true,
/*
default
{
	selector: "img",
	strength: 1,
*/
});

// or 
var ig = new eg.InfiniteGrid(container, {
	parallax: {
		selector: ".thumbnail", 
		strength: 1, // default 1
	},
});

Image is longer than the area(item)

.item {
	height: 400px;
	overflow:hidden;
}
img {
	height: 600px;
}

Surrounding area is images's parent node.

<div class="item">
	<img src="...." class="thumbnail">
</div>

Parallax scrolling use transform level 1.
This is supported from IE9 or later, but use with a prefix.