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
Modular re-write of sketchpad runtime (#220)
- Contingent on canvas changes that are still to be comitted
- Breaks GUI
Restructure back-end framework
- For now only legacy canvas back-end
- Use yamldoc docstrings
- User factory function instead of class morphing
- All back-ends based on abstract classes (not legacy)
Baby steps towards unittest framework
All canvas back-ends now inherit canvas
Convert keyboard and mouse back-ends
- Use base classes
- Use function-style factory
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
Also accept notes ('a1', etc.) in synth
Fully restore runtime functionality