@zamiang => Prevent flooding recipients with SMS messages.

This uses memcached to store which numbers have been sent messages to for a few minutes. Then displays an annoying message if you try to resend it.

![screen shot 2014-09-10 at 6 55 27 pm](https://cloud.githubusercontent.com/assets/542335/4226985/9dc87856-393d-11e4-8b88-07cd6dc1e65d.png)

I tried to make this global somehow, but I had hard time stubbing it in tests. Also I don't think it matters.