Replaces sendmail with direct

Closes #1538
* rips out sendmail
* adds direct
* updates nodemailer to 0.7.1
* changes unit tests for mail
* changes integration tests for mail
* adds api tests for mail
The rest of that links to how to set up Ghost's email, and it tells people that it's recommended to do that. I can potentially tell them they're doing it wrong though. The docs (http://support.ghost.org/mail) do not mention sendmail or direct at all, so linking them there to find out why it's a bad idea is useless at the moment.