Change IS to cache HTML instead of nodes.

This patch teaches Infinite Scroll to cache the HTML
from the xhr response instead of caching actual DOM nodes
when yanking unseen pages from the DOM. This improves the
memory usage of the browser tremendously, especially in
pages that contain many memory intensive DOM nodes, like images.

Compared to the previous strategy of caching actual DOM nodes
in a set of DocumentFragments, this technique allows the GC to
collect the memory used to back large DOM nodes.