key_length and confirmed emails

So, first of all, I wanted to thank for a great app.
Couple things I thought could be useful to add
* ability to add confirmed emails in case they were confirmed in some other way (django-registration for instance)
* `key` field's `max_length` is set to 40, but there is no way to override length of generated key without  customizing the `EmailAddressManager`
So this pull request addresses both issues.
All tests pass, but no new tests was added.