Set EmailUser.Meta.swappable to 'AUTH_USER_MODEL'

Without this set creating a custom user from AbstractEmailUser models
would not validate when using Django 1.6c1. Django 1.5 was no problem.
Should this reported up stream?

Signed-off-by: Simon Luijk <simon@simonluijk.com>
Merge pull request #1 from simonluijk/master

Set EmailUser.Meta.swappable to 'AUTH_USER_MODEL'