Wrapspawner/JH0.9.4 compatibilty: get_current_user and current_port

[rkdarst]

This fixes two problems:

First is a compatibility for JupyterHub later than 0.9.4 (but not inclusive). get_current_user became async, and thus we need to handle both old and new hubs. This adds some logic to use the non-async version to avoid making this whole method async - because jupyterhub.util.maybe_future is also relatively new so would need backwards compatibility to use itself. Closes: #131

Second, wrapspawner has to proxy the self.current_port setting to self.child_spawner.current_port. It would be better to solve this in wrapspawner, since many different things could be set. But this might solve the problem faster. Closes: #129, #127,

These are together because they touch lines almost identical to each other. They can easily be split.

[rkdarst]

Closed in favor of #143 and other PRs which are mentioned there.