Nox

* On Windows, use the `py.exe` [launcher][2] (e.g. `py.exe -2.7`) to locate
Python executables. ([53][1])

* Fix Nox not returning a non-zero exit code on failure. (55)
* Restore result and report output. (57)

* Blacklist problematic virtualenv environment variables (49)
* Use `python -m virtualenv` to invoke virtualenv (47)
* Making sure all `Command`s use run in `__call__`. (48)
* Addition of `session.notify`. (39)
* Refactor the list of sessions into a manifest class. (38)
* Changed some instances of session to be plural (37)
* Small documentation updates (36)

[1]: https://github.com/theacodes/nox/pull/53
[2]: https://docs.python.org/3/using/windows.html#launcher