Tweaks for #13

Efficiency update to nest URL constructor into if statement that tests
browser support for new pushState/replaceState HTML5 method.

Changed from pushState to replaceState method. Both accomplish the same
thing, replacing the URL without reloading the page. They differ in that
pushState retains browsing history, whereas replaceState removes the
most recent item from history.

In this case it's preferable to replace the most recent item in the
browser history, so visitor using the browser's back button will not
encounter the URL with the hash tag (and can't copy that URL for use).