Added ability to define a class for pagination unordered list element

This pull request adds a parameter to pagination to allow the `<ul>` to have a class defined.

Currently, the pagination displays like this:

```html
<ul>
    <!-- Pagination Items -->
</ul>
```

This commit adds another parameter option to allow the following:

```php
<?php echo osc_pagination(
    array(
        'list_class' => 'pagination'
    )
); ?>
```

which results in this output:

```html
<ul class="pagination">
    <!-- Pagination Items -->
</ul>
```

This is incredibly useful for custom theming. In particular, for Zurb Foundation which relies on the pagination `ul` to have the `pagination` class.