To compatible with turbolinks

When [Turbolinks](https://github.com/rails/turbolinks) is enabled and when a link is clicked, `Turbolinks` will fetch new page and replace current page's body content with new page's one. This will cause `<div class='notifyjs_corner'>` disappeared from DOM, and notify message will never be showed any more.

This patch will add a method to clear globalAnchors to be compatible with javascript library like `Turbolinks`.
With `Turbolinks` it may be invoked like

```
$(document).on("page:load", function(){
  $.notify.clearAnchors();
});

```