Agent IIFE for utils

First step towards getting agent off of `this` and onto it's own object.

If https://github.com/marionettejs/marionette.inspector/pull/162 is accepted all of the agent/utils that were attached directly to `this`, will now be attached to `this` via the Agent IIFE

Once everything is similarly wrapped we can swap the IIFE's loading `this` to a `var Agent = {};`