Drawing layer should work when exported to HTML

[pbugnion]

At the moment, the drawing layer only works in the notebook. We should see if it's possible to move more of the interaction client-side, so that the map still works when exported to HTML.

The main source of problems will be widget instantiation: when a feature is added to the map, this sends a callback to the Python side, which instantiates an appropriate widget. We need to replace this with an instantiation of the widget client-side.

Originally opened in issue #223 .