Retry addUser if email taken, closes #547

Basically tells you that the email is taken, and asks you to enter a new email address.

![screen shot 2014-09-12 at 12 34 25 pm](https://cloud.githubusercontent.com/assets/34726/4252987/b5de3f12-3a9a-11e4-9b05-378f753d84f5.png)