Switch from nose tests to pytest.

All fairly straightforward except the fsfix mess that
pytest supports without the attach_teardown() hack.
Nuke unused torndown function
Update other toolage and docs

Switch travis to do a 'make analyse'
Remove nosetest reference from docs
get rid of 'go-tdd' convenience script
get rid of tox config since we don't use it or know how to maintain it
Merge pull request #230 from gittip/issue226

Switch from nose tests to pytest.