tox.ini: Add {posargs} to py.test invocation

This allows you to specify args on the tox command line that get passed through
to py.test -- e.g.:

tox -e py27 -- -k testResolve --tb=short setuptools/tests

tox.ini

@@ -2,4 +2,4 @@
 envlist = py26,py27,py31,py32,py33,py34
 [testenv]
 deps=pytest
-commands=py.test
+commands=py.test {posargs}