Mailman

This PR introduces the Mailman, via mailman.js!  Thanks to an open source npm module 'nodemailer', we can send emails via our Gmail account.  By the way, because we can send emails programmatically, Google sent us an email to JRR saying that our Gmail account is less secure.  I think that we should eventually make another Gmail address whose sole purpose is to serve as an email sending endpoint, for greater security.

The first commit is our code, the second commit should just be the 'nodemailer' module.

After this PR, email messages will be sent to email addresses / people when a conversation is created, and to active participants when a new post is created in a convo that they are involved in.

These changes do not address what happens when you invite someone to a conversation using their email address if the recipient hasn't yet registered with GC.  Nor does it address having settings for preventing email notifications.

Also, this code won't work locally until you copy the secret.js file I sent you guys into your root goldenconversations/ directory.  I'm hoping that PR #15 will get merged in first, after which, the credentials could be further separated for this PR.