Postel rocks

This pull request supersedes the other, which is is for proof of concept purposes.

With this pull request, we introduce lax processing for messages. For any header required by brighter

* If the header is present and parseable, use it.
* If the header is present but not parseable, return MT_UNACCEPTABLE
* If the header is not present, default it to a sane value and log a warning.

We use the routing key as the topic, which may not play well with other exchange types. I'm expecting this PR to be poked and played with before merging, it's just a place to capture the conversation.