TokenStore - Define init/1 required by behaviour GenServer

[nitinstp23]

Fix for this warning message -

warning: function init/1 required by behaviour GenServer is not implemented (in module Goth.TokenStore).

We will inject a default implementation for now:

    def init(args) do
      {:ok, args}
    end

But you want to define your own implementation that converts the arguments given to GenServer.start_link/3 to the server state

[nitinstp23]

Fix #37