Add Gamepad API.

[mseddon]

https://www.w3.org/TR/2015/WD-gamepad-20151217/

[sjrd]

Most of these things should be top-level in the package, rather than in the package object.
Basically everything but the implicit pimp of Navigator.

[mseddon]

Doh, good point. I kept the GamepadMappingType literals as final vals in the package though, for lack of a better place to put them (perhaps in a Gamepad companion object, if you feel they should not be in that package).

[sjrd]

This must extend js.Any.

[sjrd]

That's all.

[mseddon]

Updated.

[sjrd]

LGTM