Generate an actual file by spawning gst-launch; otherwise
with proper error handling we get an error from wavparse that
there is not enough data to typefind.
* morituri/common/gstreamer.py:
Move the gst import to start() and set it as a class attribute.
Document methods.
* morituri/common/encode.py:
Convert EncodeTask to a GstPipelineTask.