Argus history integration

Argus recognizes when History.js (modern browser history management) is available and moves through the tree by updating, and then responding to, the browser's history. This nicely unifies tree locations and some view settings with URLs, to support emailed URLs, local comments, etc.

This branch also includes a number of cosmetic enhancements and rendering improvements for argus. Its view is now centered on the target node, paper size matches the actual tree height (for accurate scrolling of the argus viewport), and the anchored widgets in upper left stay in place as the viewport scrolls.