Smart close: check if we're destroying the right instance

[xfra35]

Since the plugin handles only one modal at a time, the need may arise to have one modal:close trigger another modal:open in order to implement state management. Here's an example:

$modal1.on('modal:close',function(){
  $modal2.modal();
});

This doesn't play nicely with the current codebase: $modal2 shows up but stop responding afterwards (uncloseable). The reason is that the current instance gets destroyed by the first modal close() call.

This PR intends to fix that by adding a simple check before destroying the current instance.

NB: Unsubscribing to keydown.modal is performed a bit earlier for the same reason: this.hide() may trigger another modal:open.