Dev workflow improvements

Replaces the repl-helper namespace with a namespace in user.

Design closely follows @stuartsierra ideas on
http://thinkrelevance.com/blog/2013/06/04/clojure-workflow-reloaded

Defines the current dev system as a naked var instead a Delay object. Alters the
var's root value when the system changes state.

Also adds attempt to close the database connection on system stop.