Add setting for primary email holder when sharing email addresses

Fixes #36 

Kinda wordy, but not sure how to shorten without losing clarity. Input welcome.

![screen shot 2014-09-23 at 10 37 27 pm](https://cloud.githubusercontent.com/assets/669/4383230/325e7b38-439c-11e4-8e57-4f2b5ed42e6b.png)