Add email field to User model (#312)

- Add an email column (to model, not unique, migration)
- Populate email with 'X-Email' header at submission time
- Update tasks to handle
- Always expect a nickname, optionally populate email in user object in process_user
- If email addresses don't match for existing user, update email
- Add tests

@hannosch 
The model definition says Unicode(128), the migration String(255) - which one do you want? I'd say go with Unicode(255) :)