Setup dependency on pytest-runner makes installation hard in the walled environments

[Alex Bason]

setup.py specifies setup_requires=['pytest-runner'] which, if not present, would be installed using EasyInstall which wouldn't respect any pip options, like custom index URLs.
As a workaround, pytest-runner documentation suggests using conditional dependency. Alternatively, that dependency could be completely eliminated (make test runs pytest anyway).

[Nathaniel J. Smith]

I'd be fine with dropping this... setup.py test is a neat idea that never really took off. And all the other python-trio projects recommend calling pytest directly anyway. @smurfix I think you set this up – any opinion?

[Alex Bason]

I'd suggest switching to tox to cover different versions of python (any maybe trio too). Drafted it in #48.

[Matthias Urlichs]

Merged.