use pytest instead of nose
Fix docs and CI script (use py.test)
Fix doctests

Use generator based test case behavior (this should work in pytest as
well as in nose) instead of `DocFileSuite` quirk, as the latter doesn't
work with pytest.

This is a bit of a hack (though not uglier then before) and should be
replaced by proper pytest doctest support.
Merge remote branch 'origin/feature-343-pytest' into develop