tox.ini: break up test run command for readability

The main test run command is quite long. This breaks it into multiple
lines in the tox.ini file for better readability.
tox.ini: add {posargs} to test run command

This is useful because you can inject additional pytest flags and
arguments (for running in verbose mode, running specific tests, etc.).

So now, any of the pytest invocation options
(http://pytest.org/latest/usage.html) can be passed to tox, like so:

tox -e py27 -- -x -k test_foo
Merge pull request #55 from larsbutler/posargs-for-pytest

Add {posargs} for tox test run config