Change default screenshot_dir to avoid writing to /

The current default screenshot_dir value of '.' results in screenshots
being written to the root directory.

> OzException: No disk activity in 300 seconds, failing.  Check screenshot
> at /client-a14da48d-ef38-485a-afb7-56f69597d553-1326467720.58.png for
> more detail

This patch changes the default to '/tmp/oz-screenshots' and creates the
directory if needed while saving a screenshot.