move commonly used util functions out of core

[caolan]

eg, getDBURL() is inside core, but core will require the app making a circular require problem. To avoid this, move any commonly used public api into duality/utils or some other module.

Its annoying having to do var appdb = db.use(duality.getDBURL()) inside each list/show function instead of at the top of the module. Hopefully this can be fixed by moving things around a bit.