Add validator

This PR adds a validator to ease the use of Akismet in data validation.

Usage :

    use Ornicar\AkismetBundle\Validator\Akismet;

    /**
     * @Akismet(author="auteur", content="contenu")
     */
    class Commentaire
    {
        private $auteur;
        private $contenu;
        // ...
    }