feat($location): add support for state and title in pushState

This is a continuation of #3325, re-created per my e-mail thread with @IgorMinar.

Adds $location pushState & replaceState methods acting as proxies to history
pushState & replaceState methods. This allows using pushState to change state
and title as well as URL. Note that these methods are not compatible with
browsers not supporting the HTML5 History API, e.g. IE 9.
The `equals` is needed to prevent an infinite digest loop, otherwise the counter would never stabilize. The `$location.state() != newState` check is needed to merge `undefined` and `null` (see my previous comment) which is not covered by `equals`.