Invitation mode, for use in conjunction with django-invitation

Hello, 

I made some changes to add an INVITE_MODE setting to be used in conjunction with django-invitation, so that users are only activated if they have received an invitation code.  

The case where a user creates an account without an invitation code, and then subsequently receives an invite code and logs in again, does also rely on a change I just submitted to django-invitation.

Let me know what you think,
Liz