remove sudo

Add dedicated SudoSpawner in a [separate repo](https://github.com/jupyter/sudospawner), which works much better than this ever did by adding a mediator script that actually gets run as sudo, rather than trying to directly start the single-user server with sudo, which is a huge pain.

@dsblank can you give the sudo spawner a try? I know you are using the sudo config. Hopefully this should be an improvement.

closes #101
closes #102