Bugfixes, Improvements

Fixes an issue with the calling of Arpk.exit() from the debug terminal
session.

This commit improves the performance of shutdown, window cleanup, and
ObjGraph method introspection.

Also included is a revision to the DB mixin which allows for the
specification of the table, primary key, data key, and type key names
used in all precomposed queries (see: store_* and retrieve_* methods).
These values may be specified at the time of connection via the options
argument, or afterwards via their analogous modifier methods.