add ishtar_listener:info/1, ishtar_listener:info/2

    1> ishtar_listener:get_port(http).
    {ok,8080}
    2> ishtar_listener:info(http).
    [{ip,{0,0,0,0}},{port,8080},{open_reqs,0},{acceptors,100}]
    3> ishtar_listener:info(http, [port]).
    [{port,8080}]