Invite-Only System 0.3.1

Invite-Only system in which guests need an invite from forum members to access the forum.

Sponsored by ADM.

Change log:
+ 0.3.1:
- Fixed installer bug when the invitation fields in the user table exist already.
- More friendly link (thanks to gpreston).
+ 0.3.0:
- Fix CSRF vulnerability.
+ 0.2.2:
- Correct a bug with the forum presentation option.
+ 0.2.1:
- Correct a bug in forcing enabling whispers.