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.

```
$ tox
...
  py26: commands succeeded
  py27: commands succeeded
  py33: commands succeeded
  py34: commands succeeded
  pypy: commands succeeded
  congratulations :)
```