Added gst _sampler and _synth backend

GStreamer can now also handle sound requirements of sample and synth
elements

Conflicts:
	openexp/_sampler/gstreamer.py
	openexp/_synth/gstreamer.py
Updated docstrings

All docstrings now simply point to those of the legacy backends
Add temporary xpyriment-gst backend setting
Fixed freezes on Linux

changed self.experiment.sleep() calls to time.sleep() as the latter
appear not to mess up multithreading
Merge branch 'playground' into soundbackends
Merge branch 'soundbackends' into heisenberg-sketchpad
Initial pass at full bvack-end conversion to new framework
Unbreak the GUI
- sketchpad and feedback controls removed, pending re-implementation