Mailgun receive support

Summary:
As you've suggested, I took the SendGrid code and massaged it until it played nice with Mailgun.

btw - unless I'm missing something, it appears that the SendGrid receiver lets you spoof emails (it performs no validation on the data received).

Test Plan: Opened a task with Mailgun. Felt great.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4326

Differential Revision: https://secure.phabricator.com/D7989