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
Merge branch 'soundbackends' into playground

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