Have tox install via setup.py develop

Tox 1.6 has a feature to install code into the virtualenv via 'setup.py
develop' This makes it easier to source the tox virtualenv and test code
changes.
Make _flake8_noqa tighter

Only match '\s*# flake8[:=]\s*nqa' when it happens at the beginning of a
line. The previous version of this regex is incorrectly hitting the
following line:

        # Any "# flake8: noqa" line?

Causing flake8 to not be run on engine.py.
Also fix flake8 issues in engine.py.
Loosen the regular expression search

I have seen people who place the comment at the bottom of the file and it
would be backwards incompatible if we did anything but preserve it as a search
instead of match
Merge branch 'setupdevelop' into 'master'

Add usedevelop to tox.ini

See merge request !5