Reorganize fas shim to:

* Support gravatarizing an email, not just a username.
* Support libravatar like python-fedora will some day.
Mailman3 processor and test.
PEP8 and corrections.
The "references" header actually specifies whether or not it is a reply.
And... references can be None.
Another test for mailman.
Emit a warning and don't TB when handling an invalid mailman3 message.
Check both "references" and "in-reply-to" headers when determining mailman3 subtitle.
Modify test to handle both cases.
Handle that dichotomy in the mailman .objects method, too.
Merge pull request #18 from fedora-infra/feature/mailman3

Feature/mailman3