Asking from email to users that doesn't have it yet.

 - [x] spec that an authenticated user without email will be asked for it when creating events
 - [x] spec that an authenticated user without email will be asked for it when creating proposals
 - [x] configure the devise confirmation when an email is given
 - [x] consider emails retrieved by omniauth (from twitter, facebook, github...) confirmed by default
 - [x] to configure an email provider addon on heroku
 - [x] go to the crowd

This is part of the solution for #54 and #15.