Function factories

Adds function factories for scrollTop, scrollLeft, content height and content width. This allows performing more exotic scrolling than just scrolling the container where scrollbars are added using default browser scrolling support, e.g.: scrolling of iframe inside a container.
I am planning to use this functionality in widget-it (https://github.com/cmanzana/widget-it), which will be a jquery plugin that allows creating widgets of existing web pages using iframes.