Start of the msg2long_form API.

This will eventually be used by bodhi and FMN to send out bigger, more
informative blocks of text.

For fedora-infra/fedmsg_meta_fedora_infrastructure#96
Use assertEquals for nicer output.
Even better.
Merge pull request #287 from fedora-infra/feature/long-form

Start of the msg2long_form API.