add global "elog" function to emonk

This functions allows you to natively report logs to the erlang
error_logger:

    1> application:start(lager).
    ok
    15:32:32.695 [info] Application lager started on node nonode@nohost

    2> {ok, Ctx} = emonk:create_ctx().
    {ok,<<>>}

    3> emonk:eval(Ctx, <<"elog('test'); var f = 2; f*3;" >>).
    15:32:47.984 [info] [js log] test

    {ok,6}