Based on first review by @minrk
* bundlerextension for consistency
* Move CLI entry point into bundler package
* Add bundler/__main__.py
(c) Copyright IBM Corp. 2016
* Change handler for consistency with nbconvert
* Change bundler function API to take ContentManager models
* Change CLI to work as jupyter bundler
* Change UI to fit into existing template / JS structure
(c) Copyright IBM Corp. 2016