[Proposal] Provide abstract methods to create a custom paginator presenter

Current version of the `BootstrapPresenter` doesn't allow to extend the HTML codes of the pages. This proposal provides some abstract methods to create a custom presenter easily. Current `BootstrapPresenter` in the proposal is actually a custom presenter.

**An example view:**

```php
<?php
// An example custom presenter for Zurb Foundation
class ZurbPresenter extends Illuminate\Pagination\Presenter {

	public function getActivePageWrapper($text) 
	{
		return '<li class="current">' . $text . '</li>';
	}

	public function getDisabledTextWrapper($text) 
	{
		return '<li class="unavailable">' . $text . '</li>';
	}

	public function getPageLinkWrapper($url, $page) 
	{
		return '<li><a href="' . $url . '">' . $page . '</a></li>';
	}
}

$presenter = new ZurbPresenter($paginator);
?>
<ul class="pagination">
<?php echo $presenter->render(); ?>
</ul>
```