VNC backend update

- Add IPython DOMWidget to _ipynb_vnc. Now, backend has its own widget to display.
- Edit setup.py, .js files will appear in build.
- No need to write Js code into notebook for event handling.
Based on the conversation between IPython devs, Thomas Kluyver's response was:
http://mail.scipy.org/pipermail/ipython-dev/2014-July/014383.html
So I thought it was the right thing to do.