Pytest-xvfb

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- Use `PyVirtualDisplay`_ to start/stop Xvfb
- Show a warning on Linux if Xvfb is unavailable

.. _PyVirtualDisplay: https://pypi.python.org/pypi/PyVirtualDisplay

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- Add a new ``xvfb_xauth`` setting which creates an ``XAUTHORITY`` file.

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- The temporary directory searched for logfiles is now hardcoded to /tmp
as that's what X11 does as well.

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- The ``no_xvfb``-marker is now registered automatically so pytest doesn't fail
when run with ``--strict``.
- The ``xvfb`` fixture is now session-scoped.

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- Initial release