Do not except on special keys (such as 'volume up') and do not assume `.flush()` methods on standard output.

I encountered two minor issues. They are not so much bugs in PsychoPy as they are safeguards to deal with issues in other software.

- Catch `ValueError`s that occur in PyGlet when you press special keys, such as `volume up`, etc. This may be specific to certain configurations, but it certainly occurs on (K)Ubuntu 13.04 with PsychoPy 1.77.01.
- Do not assume that streams have `.flush()` method.