Wall-to-wall example

A few months ago @brendancol created a very nice example of running Datashader full screen with data covering the entire area, which is great for exploring data:

https://github.com/brendancol/simple-react-map

Here he runs a Python server for Datashader in one process in a terminal:

```
python server.py
```

and runs a JavaScript-based client (using React and Leaftlet) in another:

```
webpack-dev-server
```

Now that HoloViews supports Bokeh Server, I think it should be feasible to port this application to a pure Python approach, though I believe that would require packaging up a few key bits of JavaScript as Python objects. This seems like a good way to show off what's possible with Bokeh Server + HoloViews + Datashader...