Add start_link/1 function to GroupMemberSupervisor

[objectuser]

This function is in support for the new supervisor child specs supported in Elixir. This allows just the module to be provided as a child to the parent supervisor.

  def start(_type, _args) do

    children = [
      Kaffe.GroupMemberSupervisor
    ]

    opts = [strategy: :one_for_all, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end

[dtykocki]

@objectuser - Is this something that's still worth merging in? Are there any risks with backwards compatibility?

[objectuser]

@dtykocki I'm not sure why I didn't merge this in. I don't think there are any compatibility issues because it's additive. I do think the extra changes should be removed.

[vittoriabitton]

That change can be merged to be updated with the Supervisor child specs?