Add Mailgun support.

This commit includes a MailgunRequestParser implementation. Mailgun
uses the concept of 'routes' to send inbound emails, and does a lot
of really useful heavy lifting server-side - the content is all UTF-8
encoded, and the strip out the cruft from message replies.
Update MIDDlEWARE_CLASSES setting to appease Django 1.7

> HINT: Django 1.7 changed the global defaults for the MIDDLEWARE_CLASSES.
> django.contrib.sessions.middleware.SessionMiddleware,
> django.contrib.auth.middleware.AuthenticationMiddleware,
> and django.contrib.messages.middleware.MessageMiddleware were removed
> from the defaults. If your project needs these middleware then you
> should configure this setting.
Merge pull request #14 from yunojuno/feature/mailgun

Add Mailgun support.