Add email creds

@scwu @adelq 

This should fix #193. I've already updated the prod droplet with Sendgrid params (which Django will use accordingly). But I don't know if it will work yet as Sendgrid is still provisioning and can't send e-mail. I added the sendgrid creds to the passwords doc — you should get an e-mail at pennappslabs@gmail.com when they're done, at which point you should test creating new users and makes sure it send the registration e-mail properly