Lots of docs and other changes

Simplify conf.py; use RTD theme; separate docs requirements;
update contributing docs
Update setup.cfg

Exclude virtualenvs and other dirs from flaking
Install dev-requirements.txt for tox and travis

because we often put soft dependencies and such in there
Remove test command from setup.py

I never really use that. KISS
Don't put changelog in long description

Instead, link to it.
I often use sphinx-issues, so the changelog won't render correctly
on the PyPI anyway
Remove unused docs dirs