Added background functionnality

Modified responsiveImg.js to support css background. Set your css background to a spacer (so it doesn't load) 


	#header {
		background:#f0f8ff url("../images/spacer.gif") no-repeat 0% 80%;
	[...]
	}

and add attribute data-src to the html element with the large background's path.

	<div id="header" data-src="images/bg-header.jpg">

Then call something like 

	$("[data-src]").responsiveImg({ srcAttribute:"data-src" });

Modified responsiveImg.js.php: minor optimization to avoid file system overhead.