(NC-160) Allow partial configuration of db-spec.

The user can specify any subset of `subprotocol`, `dbname`, `user`, and
`password` in the database section of the configuration file, and the
rest will be filled in with default values (and `dbname` will be renamed
to `subname` for the spec).
(NC-160) Add simple example to README.

Walk the user through creating a class, creating a group that references
it, and observing the classification of a node into said group.
Merge pull request #43 from aperiodic/nc-160-update-readme

(NC-160) Update README