This reduces some of the complexity in testein.py, but unfortunately running
tests is still unreliable. Running batchwise tests don't work at all in Windows,
and running from inside emacs tests often need to be run multiple times before
they pass.
The worst offender is the delete notebook test, which will pass on usually only
1 out of 3 tries.
Testing seems to have revealed a couple bugs, so win??
When calling `ein:jupyter-server-stop` ask to user if they are sure and also
give them the option to save any unsaved notebooks before killing the server.
Before closing the server also close any open notebook buffers.
For the moment we only support current version of jupyter (i.e. 4.3.1 or
greater).
The special commands are `ein:jupyter-server-start` to start a notebook server
and `ein:jupyter-server-stop` to, clearly, stop a notebook server.
On starting a server EIN will try to determine the url and token for accessing
the notebook server, login and open the notebook list automatically.
