A second take on the 'conglomerate' API.

The first approach in #278 didn't provide enough support for what I
ended up needing to implement in fedmsg_meta_fedora_infrastructure.

The gist here is that fedmsg core will provide a ``BaseConglomerator``
class that plugins will extend.  It is an abstract base class, and
children will have to override three different methods for it to work
properly.

I already have a functioning Conglomerator for some bodhi messages that
I'll submit as a PR soon.
Add a convenience method for subclasses.
Be more explicit with warnings from processors.

Should have done this a long time ago.
Fix class/object attribute collision.
Pep8.
Merge pull request #279 from fedora-infra/feature/grouped

A second take on the 'conglomerate' API.