Add Mailgun support

Summary:
This change adds a service provider and a Mailgun facade to
be able to retrieve a message stored on Mailgun servers.

This will allow the notifications center to receive notifications
by e-mail through Mailgun inbound mails.

Ref T1000.

Test Plan: New unit tests for facade and service provider.

Reviewers: dereckson

Subscribers: alken-orin

Maniphest Tasks: T1000

Differential Revision: https://devcentral.nasqueron.org/D640