Marrow.mailer

* Automatic `Message-Id` assignment if missing during MIME message generation.
* Corrected testing issue under Python 3.
* Final release expected to support Python 2.

This is a minor bump of Marrow Mailer addressing the following:
- Somewhat modernized testing harness.
- 61 Corrections for SES issues.
- 64 Documentation updates. (Also: 68)
- 67 Python 3 corrections.
- 69 Better handling of attachment filename encoding.

This release introduces support for additional transports:
- Mailgun
- SendGrid
- Postmark