Feature: resend invites

Clicking on *Resend Invite* button will update the `Invite.created_at` date to `Time.zone.now` and will send an invitation email (with the same invite key).

Example:

![invites](https://cloud.githubusercontent.com/assets/5732281/4533066/269a92c6-4d99-11e4-8454-8da14afbe4d8.png)