added default email (user.email) value for adressess

[restless]

What it does:

Normalizes usage of initials for adress forms. If user is authenticated then value of user.email is automatically placed into the address forms (invoice & shipping) on the checkout page. Previously email field was empty, even if user had it set in user account.

For anonymous checkouts - nothing changed.

[diefenbach]

Thx!