Expand setup.py test/py.test options description

- Use sphinx option markup for clarity/cross-referencing
- Put -h/--help at the top of the option list
- Add description for --markers option of setup.py test
- Add description for --repeat option of py.test
Unify option description
Merge pull request #888 from sbesson/rebased/develop/pytest_repeat

Expand setup.py test/py.test options description (rebased onto develop)