Stop polling the DOM.

[jashkenas]

At the moment, we have poll the DOM every 100 milliseconds, doing two things: checking to see if the window dimensions have changed, in order to resize the viewer, and checking to see if the viewer DIV has been scrolled down, and we need to load a new page.

The polling could be removed, and the feel of resizing and scrolling correspondingly tightened up, if we can figure out a way to reliably and performantly work with scroll and resize events in a cross-browser fashion.

[jashkenas]

On the kill_ie6_support branch, the first of these two things is now done -- we now use CSS absolute positioning hard left and hard right, instead of using javascript to set the width of the viewer.