Update browser URL via pushstate API if available

With the back support we generate a REST-URL for every page and change it in the footer of the page so the user can copy/paste it. However, many users don't know this, they constantly send us the copy of the URL from the browser bar.

Recent browsers support the Pushstate API (http://caniuse.com/#search=pushstate). The goal of this issue is to push the current REST URL to the browser bar when pushstate is available.