Add community widget examples

[pbugnion]

It would be good to have one or two examples of bqplot and ipyvolume.

I think the following make for a good example:

• it should be simple: the point is to show how to use ipywidgets-server, rather than demonstrating advanced features of bqplot.
• it should use Python callbacks -- the strength of ipywidgets-server (compared to the embedding mode for widgets) is that Python callbacks work.
• it should load quickly; since we don't do kernel preheating yet, this means that we can't do very much pre-processing before displaying the widgets.