Transport impl that can be used to mock out HCI commands

Example:

iex> BlueHeron.HCI.Transport.start_link(%BlueHeron.HCI.Transport.NULL{
...>  init_commands: [Harald.HCI.ControllerAndBaseband.reset()],
...>  replies: %{
...>    Harald.HCI.ControllerAndBaseband.reset() => "\x0e\x04\x03\x03\x0c\x00"
...> }})