Handle virtualenv development mode
Looking for VIRTUAL_ENV environment variable as a sign of virtualenv
activation. This at least works for developing CubicWeb itself (not sure yet
about cubes). And it makes `tox --develop` work as well.
Add a test for cwconfig._find_prefix within a virtualenv.
In tox.ini, use `develop option`_ accordingly. This makes tox running
significantly faster since it avoids building and installing the sdist. Aside,
the "develop" install is now performed *after* all test requirements are
processed, which means that the cubicweb package should always be the one from
source -- instead of a released version pulled by a cube through test
dependencies, which may occur in "recreate" step.
.. _`develop option`: <http://tox.readthedocs.org/en/latest/config.html#confval-usedevelop=BOOL