Revert "Finally decided to remove the uniqueness of email addresses"

This reverts commit c630c56358d722aeb5c5d5427f72774a1dc7bee7.

I've talked to Mathieu (the original commit author) about this, the
owner field has not much to do with common newsletter functionality and
from my POV can be removed.
Introduce AutoSlugField
South is a requirement, after all we are using migrations
Remove last bits of Contact.content_type
Subscriber list can be empty

... especially when you first create a mailing list and/or newsletter
without any subscribers.
"Exportation" is such a bad word...
Reflect recent changes in the README
Introduce Contact.full_name

Names are vastly more complex than first_name + last_name
Automatic subscription to mailing lists
Extend README with fork information