Support GATT services over BR/EDR.

Preview at https://rawgit.com/WebBluetoothCG/web-bluetooth/support-bredr/index.html

@armansito
I think we should allow this so that UAs have the freedom to experiment when finding the "best" central behavior. We could write `MAY but SHOULD NOT`, which would allow use of an OS API that takes a list of UUIDs and makes the connections itself (Mac and Android aren't clear about their behavior), but I think we don't know enough yet to be confident of the SHOULD. I'll add it if you feel strongly.