Testing docs and doc tests #432

Originally reported by: Piotr Banaszkiewicz (BitBucket: pbanaszkiewicz, GitHub: pbanaszkiewicz)

Testing docs with tox right now requires make. Not everyone has it installed.
(And regen target for tox doesn't work, too)

I propose to expand testing docs and doctests so that:

1. rst syntax in docstrings is checked
2. make is no longed needed
3. doc trees are shared per tox env. run
4. rm -rf is not needed (lets not repeat other projects' mistakes)
5. Sphinx is more strict about warnings (-W)

I'm ready to submit a pull request if you agree.

I would also like to somehow force developers to write docs about implemented new features or changed pytest behavior.

• Bitbucket: https://bitbucket.org/pytest-dev/pytest/issue/432