[Sluggable] History of changed slugs

Hi,

I implemented ability to setup sluggable to store old slugs in the separate table when you update entity. After that you can use repository of the history entity and make a query which will return object which has the slug in the past.

E.g. if you create an article with slug *great-application*. After some time you decide to change the slug to *very-great-application*. You can load the article by old slug and redirect user to new one, which is better than throwing 404

I make this feature for Annotation driver and ORM. If somebody can make it for other drivers it would be great.