Ease subclassing : PostqueueStore.MailClass attr.

Currently, if I want to subclass `store.PostqueueStore` and `store.Mail`, I have to rewrite entire methods just to change the `store.Mail` className. That's bad for maintenance.

That proposal is to make it a class attribute, easing the subclassing.
Merge pull request #2 from JocelynDelalande/patch-1

Ease subclassing : PostqueueStore.MailClass attr.