This adds functionality for getting laughter by using jrgillick's
laughter detection library

NB python expects all of the feature extractor's dependencies to be
available; perhaps in future we can do something even fancier like
activating another python env

[retroactive commit]

This composes an Interval with some ancillary data:
 - source (what feature extractor did we get this from)
 - path (where is it now on the FS)
 - score (for ranking)

TODO double check we aren't overloading the word 'source' to have two
meanings

These are useful for changing Intervals (eg consolitation)

Consolidator:
  interface

OverlapConsolidator:
  condenses features if their times overlap

Classes for actually making video files

 - VideoProducer -- interface
 - FfmpegVideoProducer -- make a temporary video using ffmpeg

for jrgillick/laughter-detection to slot into

or at least what passes for an interface in python