Changes lost in Vision Kit code release

[drigz]

The following changes were lost when the Vision Kit code was released to the aiyprojects branch:

- Fix instructions for using the Voice Kit on Stretch (#187)
- Make it easier to turn the LED off (#167)
- Fix crash with trigger sound (#163)
- Let user code control TTS (#185)
- Enable hotwording with Cloud Speech (0b4972b4)

I have a change to restore these but I don't have access to hardware at the moment, so I can't test the changes until I'm back in the office in January.

[drigz]

Fixed by #228.