Direct support for grayscale images.

Grayscale images are interpreted by VisionBonnet as RGB where all
channels are the same. This is just an optimization to send 3 times less
data over SPI.

Change-Id: I0322cbff8381f295772b1eadead0a4d0d7f335fc

Update to Cloud Speech v1 (google#206)

The v1beta1 endpoint has been shut down, so this is necessary for using
the Cloud Speech demo now.

See google#205.

Python API and example source code for VisionKit

This commit extends the original AIY projects
python API to include drivers, APIs and demos
for the AIY Projects Vision Kit.

https://aiyprojects.withgoogle.com/vision/