Reactive bokeh

The reactive part is not there yet, since it would require more and more hacking on the JS side.
However, some improvements have landed in the DataConnector api (which has to be refactored to some better shape, since it's suffering from initial simple design)

Regarding hacking on the bokeh reactive and high level plotting, the best would be to shift this work directly to the [bokeh-scala](https://github.com/bokeh/bokeh-scala) library created by @mattpap in the @bokeh org.