md-select gets stuck open when directive is removed due to route changes

[nhayflick]

If an md-select is open when a user hits the back button or triggers a route change (clearing the view with the original md-select directive), the select gets stuck on screen with no way to remove it. Seems like there needs to be a md-select service capable of closing the select (exactly like you can do with the md-modal service) when the controller is about to be destroyed.