Added two new events to the obtaining of the cart. This triggers an even...

...t before and after the cart retrieval is completed (allows you to validate that the cart items are still valid if the cart has remained for a while for example).
There is an additional module that I have created that looks for a cookie existing and based on that cookie find a cart ID from the database and sets a session variable so that this function can retrieve a cart that persists across sessions. At the moment this is hooking MvcEvent::EVENT_ROUTE. It was make much more sense to hook this new event and set the session ID just before it's looked for. That way it's not fired every single time a route event occurs rather just when a cart is requested. That is the reason for adding this event here.