Tests for duplicate emails and informs user if this happens

Currently all other users will be added without throwing an error
In fact this doesn't even throw an error it just doesn't add the email
Closes #14 

![tumblr_mj0ge68egw1rrecxqo1_500](https://cloud.githubusercontent.com/assets/2591516/3512199/ff81beae-06b6-11e4-868b-79ffe7bc1775.gif)