tox.ini: Pin pytest==2.5.2, {posargs}, add py34

We pin pytest==2.5.2 for now, because tests fail on later versions.
{posargs} allows passing arguments to py.test from the tox command line
Add py34 as another environment to test on.
Merge pull request #27 from msabramo/tweak_tox_ini

tox.ini: Pin pytest==2.5.2, {posargs}, add py34