Add alias to BreadcrumbBuilder

[Jupi007]

Add alias to BreadcrumbBuilder. This allow to autowire the service like this:

<?php

...

use SlopeIt\BreadcrumbBundle\Service\BreadcrumbBuilder;

class AdminStayController extends AbstractController
{
    public function index(BreadcrumbBuilder $breadcrumb)
    {
        ...
    }
}

Closes #10

[asprega]

closing: this is not necessary to make the service injectable (as explained in the related issue)