ability to disable UI

[chrisdlangton]

Allow for only the requests handling without any UI indicator.
Expose a config setting to launch a function when the UI would normally change.

[zackbloom]

Everything is modular, if you include offline.js and requests.js without ui.js, you won't get a UI. If you take a peek at ui.js you should be able to get an idea of what events the UI listens to. Files: https://github.com/HubSpot/offline/tree/master/js