tox.ini: Tweak test discovery

Replace `tests/` in `addopts` with `norecursedirs`

Better to filter than to add in a default, so as not to force folks to always run all the tests.

This is motivated by the comments on another PR: https://github.com/sigmavirus24/github3.py/pull/294#discussion_r19709451

Note that the `norecursedirs` line might not even be necessary as `pytest` has fairly comprehensive defaults that might already handle the stuff that you're worried about. See http://pytest.org/latest/customize.html#confval-norecursedirs

Closes #294.