Add automation support via WebDriver

This PR integrates with the automation concepts defined in
https://w3c.github.io/sensors/#automation to allow providing motion or
orientation readings via virtual sensors through the WebDriver extension
commands defined there.

IMPORTANT: This does not mean that this specification requires
implementations to support the Generic Sensor API specification and its
derived specifications. Only the Automation section is being referenced, and
when necessary some algorithms and definitions are being duplicated here,
especially for Device Motion automation.

Device Orientation:
- The "absolute-orientation-euler" and "relative-orientation-euler" virtual
  sensor types are defined in this specification, along with a parsing
  algorithm that reads alpha, beta and gamma doubles.

Device Motion:
- Readings are controlled via the "accelerometer", "linear-acceleration" and
  "gyroscope" virtual sensor types. They are defined in the Accelerometer
  and Gyroscope specifications. A fallback path for implementations that do
  not support those specs has been provided.

Fixes #122.