tox env to build test coverage

The python module `coverage` let you easily generate coverage report. We
are using it for the mediawiki/tools/scap repository already.

Add a tox env `cover` which uses `coverage run python setup.py test` to
capture the output. Then generate a text and HTML reports (under
/htmlcov).

The new env is not invoked by default, one have to:

 tox -ecover

Then open htmlcov/index.html

Change-Id: Ia94d53be8a90ef50b3bf98c405e9c0994bc58c59