Add a means to test the API

[marcoscaceres]

The API is not currently testable for interop because it's obviously not possible always have a physical Gamepad available (and being pressed) in a continuous integration environment.

It would be great if the spec provided a Web Driver API that allowed:

• connecting a mock gamepad, and specifying or overriding a limited set of details (e.g., id and mapping).
• disconnecting a mock gamepad.
• setting button values on a mock gamepad.
• setting the axis values on a mock gamepad.
• setting possibly other supported aspects of the gamepad.

Once updated, calling getGamepads() would return the updated snapshot of the mock gamepads.

Out of scope would be to provide a means to "playback" a set of button presses.