Update jquery.responsiveimages.js

Add options for container and skip_invisible
according : http://www.appelsiini.net/projects/lazyload
Webkit browsers will report images without width and height as not .not(":visible"). This causes images to appear only when you scroll a bit. Either fix your image tags or set skip_invisible to false.

Better handling of top/bottom left/right position of element
inspired by http://www.appelsiini.net/projects/lazyload
to allow onepage horizontal layouts with jquery.ascensor.js
http://kirkas.ch/ascensor/#Home

Allow external call to $.fn.responsiveimages.update()
eg on ascensor page transition