Now plugins can define additional handlers and the handlers can be organized
into named pipelines.  Modules are no longer the unit of a handler, now we use
classes so we can assert interface conformance.

The GLOBAL_PIPELINE is gone, replaced by the 'built-in' pipeline.  The
OWNER_PIPELINE is not yet replaced.

I still need a few more tests of the basic pipeline architecture, although the
individual handlers have pretty good coverage.

Added the IHandler and IPipeline interfaces.

Still broken, but not yet removed: Mailman/pipeline/moderate.py.