Fixes #944 option to urilize relationField when using the RelativeSlugHandler

This fixes issue #944, added an extra option to urilize the relationField which is by default not done.

Reason is, that somethimes you don't want to urilize the parent field where you are relating to because the parent field is already a slug. An example is that if you have a product and a category entity, where the product entity has a relative slug to the slug of the category. In that case you don't want to urilize it (default behavior).

But when you want to relate to a parent field that is not a slug then you do want to urilize it otherwise the URL's will be incorrect. I hope this fix is appropiate.

Example: 

We have an article entity:

	<?php

	namespace ... ;

	use Doctrine\ORM\Mapping as ORM;
	use Gedmo\Mapping\Annotation as Gedmo;

	/**
	 * @ORM\Table
	 * @ORM\Entity
	 */
	class Article
	{
		/**
		 * @var integer
		 *
		 * @ORM\Column(name="id", type="integer", nullable=false)
		 * @ORM\Id
		 * @ORM\GeneratedValue(strategy="IDENTITY")
		 */
		private $id;

		/**
		 * @var \RelativeArticle
		 *
		 * @ORM\ManyToOne(targetEntity="RelativeArticle", inversedBy="articles")
		 */
		private $relativeArticle;

		/**
		 * @var string
		 *
		 * @ORM\Column(name="title", type="string", length=145, nullable=false)
		 */
		private $title;

		/**
		 * @var string
		 *
		 * @Gedmo\Slug(handlers={
		 *      @Gedmo\SlugHandler(class="Gedmo\Sluggable\Handler\RelativeSlugHandler", options={
		 *          @Gedmo\SlugHandlerOption(name="relationField", value="relativeArticle"),
		 *          @Gedmo\SlugHandlerOption(name="relationSlugField", value="title"),
		 *          @Gedmo\SlugHandlerOption(name="separator", value="/"),
		 *          @Gedmo\SlugHandlerOption(name="urilize", value=true)
		 *      })
		 * }, fields={"title", "id"}, separator="-", updatable=false)
		 * @ORM\Column(type="string", nullable=false)
		 */
		private $slug;
	}
	?>

If the title field of relativeArticle contains spaces for example, this will now be replaced with dashes. If you don't configure urilize then nothing is done.

Please review and comments / suggestions are very welcome!