Add temporary xpyriment-gst backend setting

Hi Daniel,

I played around with it for a bit and added some (temporary) info to the back-end settings so that you can easily select the gstreamer back-ends from the menu.

So far, it seems to work fairly well on Windows 7, but on (K)ubuntu 13.10 it doesn't work reliably at all. Basically, sound playback is generally mute and OpenSesame freezes when it waits until sound playback is finished. Occasionally I get a beep, but usually there's nothing. Any idea what the problem could be? Have you tested on Linux yet?

Cheers,
Sebastiaan