Add ability to expire a referral code

[paltman]

By adding a default nullable expired_at field, you could provide controls for a user to create a limited time referral link as well as controls to immediately expire a link.

[ossanna16]

@paltman It sounds like this could be a good first-timers-only issue. What do you think?

[KatherineMichel]

This was dealt with in a stale PR. Part of the PR has already been implemented through some other commit. Either conflict needs to be resolved, or new PR #13. https://github.com/pinax/pinax-referrals/pull/13/files