fix the issue with the pointer-events: none

Regarding of the message I left on https://github.com/jirikavi/AngularJS-Toaster/commit/54c599fb878de39db2996d1cc2fd50aa2c6e2a56 

This way, we can still click on the elements behind the toaster when he's still displaying (when his positionned on the top-center, bottom-center or center).