Added optional history.pushState behaviour to Tab.show.

`history.pushState` updates the browser history and address bar.

Given that bootstrap JS supports loading a tabs if their anchor text is in the url on page load, I found it unusual that the functionality of pressing tabs updating the address bar to this url was unavailable. I understand this behaviour isn't always necessary, hence the optional usage by adding `follow-url` as an attribute to the anchor which is used to show the tab.

`history.pushState` is HTML5 so wont work in older browsers, however this functionality is implemented at the end of the show method so any javascript error caused by it would't break the primary functionality of tabs. If it were desired for this to implemented for older browsers this stackoverflow post suggests a method which I haven't tested; http://stackoverflow.com/a/136506/1646387