Support for alignment and size for Bootstrap paginator

I liked that the paginator class in Laravel has support for Twitter Bootstrap out of the box but I disliked the fact that you couldn't set the alignment or size using Twitter Bootstrap. So I build it in :)

I present to you: paginator alignment en size with Twitter Bootstrap out of the box.

To set alignment when rendering with the `->links()` function:

```php
<?php echo $users->links('centered', 'small'); ?>
```

Or when creating a paginator manually:

```php
$paginator = Paginator::make($items, $totalItems, $perPage, 'centered', 'small');
```

**Alignment**
Possible values: centered, right
Defaults to no class and aligns to the left -> Bootstrap default.

**Size**
Possible values: large, small, mini
Defaults to no class and normal size -> Bootstrap default.

Note that these will only work for a paginator of type `pagination::slider` not for `pagination::simple`. I plan on adding support for aligned links for the simple paginator.