Allow option for notification to not disapear

If user initiates a notification with a ttl of false, like so:
```
var notification = new NotificationFx({
    message : '<p>Sample Notification</p>',
    ...
    ttl : false				
});
```

Then the notification will persist until the user clicks the close button.
Updated comment to make it more clear
Merge pull request #1 from crookedneighbor/master

Added option to make notification persist