Now each plugin base class (PluginBase and Handler) has an abstract
method for the main functionality it provides, forcing plugin authors to
implement those methods before their plugin can be successfully used.

Now Handlers are correctly compared based on their confidence, so that
the one with the highest confidence is first to be retrieved from the
minheap.

No reason to have them in a module of their own.  And oh, the new
Handler base class exists, and the PluginBase has been revised as per
issue #18.

As part of the effort for resolving #12, I've started work on a plugin
API for Kaylee.  While very much a work in progress, it will allow
Python plugins to be written, loaded from user configuration, and
hooked in to events from necessary portions of Kaylee to handle voice
commands.

Currently there is only one plugin, a partial implementation of shell
command support as existed previously.  It works in that it executes
commands, but several old features are missing.  Also, the GUIs are
probably broken, but I'm not worried about that at the moment.

That's "Kaylee Voice Command" if you didn't figure it out.  I think
everything still works properly, but I'll do more testing later to
verify.