Add a use case for the control channel for audio devices.

@carybran, how's this look? Are there any better examples I should include?

Demo at https://rawgit.com/WebBluetoothCG/web-bluetooth/audio-use-case/use-cases.html#usecase_audio_control

Note that I'm also adding support for GATT-over-BR/EDR in #27, which should make this use case work better for your devices.