Support GATT services over BR/EDR.

Preview at https://rawgit.com/WebBluetoothCG/web-bluetooth/support-bredr/index.html

@armansito
I'm inclined to try to skip any mention of ATT in this document if that's possible. BT seems to define all the procedures we need in the GATT chapter, so we might not need to complicate people's mental model with ATT.

For this sentence, maybe `provides most of its functionality through the <a>Generic Attribute Profile (GATT)</a>.` with a link down to the description of Services, etc. below.

Think that's reasonable?