[Timestampable] multiple values with on="change"

This PR enables to watch multiple values for a field with on="change"

    /**
     * @MongoDB\Date
     * @Gedmo\Timestampable(on="change", field="state", value={"Finished", "Error"})
     */
    protected $finished;

it's only a minor change, but it opens many new use cases. Tests + addition in the docs included.