Oven virtual device sample initial commit. Used as weave device for CES
demo.

Virtual oven implements temperatureSetting, temperatureSensor, and
brightness traits.

Change-Id: I33ec5c841ad102377166086668cbb0e1b78755dc
Reviewed-on: https://weave-review.googlesource.com/1867
Reviewed-by: Peng Ying <pying@google.com>