Multiple containers

[embray]

Currently the script that launches the notebook uses just one hard-coded container named "sagemath-windows". If this container is already running the launcher doesn't even work quite properly, so the launcher should check if that container is already running and simply open the web browser if so.

Furthermore, I'm working on making it possible to run sage from the command-line as well. Currently that just reuses the "sagemath-windows" container, which doesn't work if that container is already running. Each command-line sage session should use its own container that is automatically deleted when exited--this would give a fairly transparent experience of having a "native" installation.