Implement PNotify stack

Hello,

I implement the stack configuration for my usage : http://sciactive.com/pnotify/#stacks and it's always good to share ;)

This is a sample configuration :

```javascript
/**
 * Configure notify provider
 */
angular.module('app').config(['notificationServiceProvider', function(notificationServiceProvider) {
    notificationServiceProvider.setDefaults({
        styling: "bootstrap3",
        history: true,
        delay: 40000,
        closer: false,
        closer_hover: false,
        mouse_reset: true,
        icon: true
    });

    // Configure a stack named 'bottom_right' that append a call 'stack-bottomright'
    notificationServiceProvider.setStack('bottom_right', 'stack-bottomright', {
        dir1: 'up',
        dir2: 'left',
        firstpos1: 25,
        firstpos2: 25
    });

    // Configure a stack named 'top_left' that append a call 'stack-topleft'
    notificationServiceProvider.setStack('top_left', 'stack-topleft', {
        dir1: 'down',
        dir2: 'righ',
        push: 'top'
    });

    // Setup the default stack to use with notification service methods
    notificationServiceProvider.setDefaultStack('bottom_right');
}]);
```

This is a sample usage :

```javascript
// Use default stack
notificationService.info('Hello World : Bottom right');

// Use top_left stack
notificationService.info('Hello World : Top left', 'top_left');
```

Mehdi