[web-bluetooth] Pull Request: Expose all interfaces with user-visible instances, and make sure they start with "Bluetooth".

jyasskin has just submitted a new pull request for 
https://github.com/WebBluetoothCG/web-bluetooth:

== Expose all interfaces with user-visible instances, and make sure 
they start with "Bluetooth". ==
This allows users to add helper functions to their prototypes without 
getting an
object first. This spec claims the window.Bluetooth* namespace.

I've included `Bluetooth`, the type of `navigator.bluetooth`, for 
symmetry, even though users can just modify `navigator.bluetooth` 
directly.

Demo at 
https://rawgit.com/jyasskin/web-bluetooth-1/expose-the-right-interfaces/index.html.

See https://github.com/WebBluetoothCG/web-bluetooth/pull/129