Spec changes for permission API and Eddystone integration

I have a couple changes to the spec to:

1) Adjust to recent changes to the Permission spec:
https://github.com/WebBluetoothCG/web-bluetooth/pull/249. I think the most
interesting part of this change is that it leaves device ID generation much
more up to the browser:
https://rawgit.com/jyasskin/web-bluetooth-1/update-permissions/index.html#note-device-id-tracking.
The thought was that the choice of when to forget devices is much more a UI
decision than something the spec should nail down. I expect browsers to do
something sensible with this freedom, but if folks want to refine things,
definitely speak up.

2) Integrate better with the Physical Web flow:
https://github.com/WebBluetoothCG/web-bluetooth/pull/230 (which also
includes #249; just look at the third commit). This exposes a
navigator.bluetooth.referringDevice field, which will hold a
BluetoothDevice when the page was opened from a beacon's advertised URL.

Please chime in if you think aspects of those changes could be done better.
:)

Thanks,
Jeffrey