New operator: sleeper to simulate network delays

and blocking or slow operators. sleep(delay=1.5,s) waits 1.5 times the
duration of a frame at each get_frame. Expect catchup ahead!

This is meant for testing operators (buffers in particular) and not at
all for production.
New operator: buffer.adaptative

This is just like buffer, excepting that the output reads the buffer at
slower or faster speed in order to keep the buffer at around the same
level always. For clocks a bit too slow (typically soundcard) it should
help without being noticeable. For now the operator forgets about tracks
boundaries and metadata (but they could come back).
Sleepers can die.
Add comments and averaging parameter.
Merge branch 'master' into resampled-buffer
Cosmetic.
Add support for metadata in buffer.adaptative.
Implement remaining.
Freeze is better terminology than die.
Correct negative offset problem.
Merge pull request #131 from savonet/resampled-buffer

Resampled buffer