add ability to set the notification duration



Add ability to set the notification duration and the persistence.

This is just a fix from https://github.com/DerekRies/angular-notifications/pull/8 (thanks to @pdf)

Usage:

$notification.info(title, content, userData, duration);

**Duration** is optional (milliseconds): It allows you to set how long the notification is shown. Put `0` or` false` if you want a persistent notification. If not given it take the value in the setting .

Thanks!