Add a `clean` target in the Makefile
Add a `clean` target in the Makefile

The `clean` target removes unnecessary *.pyc files, the build
and distribution folders, and the virtual environment.

Make the `test` target depend on `clean`, to ensure that we
always test in a newly created environment.

Change-Id: Iab8fb4a83cd14671c5a6bd0ac029c12349c1aa3b

gferon/pygerrit2,sonyxperiadev/pygerrit,markon/pygerrit2,morucci/pygerrit,dpursehouse/pygerrit,dpursehouse/pygerrit2,benjiii/pygerrit