Merge pull request #293 from msabramo/bring_tox.ini_from_1.0-alpha_branch

tox.ini: Bring over changes from 1.0-alpha branch
tox.ini: Use py.test directly

instead of `python setup.py test`.

The advantage of using `py.test` directly is that you can now pass
`py.test` options to take advantage of all of its neat features for
controlling test collection, output control, pdb, etc. E.g.:

    tox -e py27 -- --tb=short --pdb --pyargs tests.integration.test_repos_release -k test_download