For the following reasons:

- Ruby compatibility.
- Add UTC offset (time zone informations was not included by #4433).
- More readable.

Example:

  Before this patch:

    p Time.gm(2003,4,5,6,7,8,9)        #=> Sat Apr  5 06:07:08 2003
    p Time.local(2013,10,28,16,27,48)  #=> Mon Oct 28 16:27:48 2013

  After this patch:

    p Time.gm(2003,4,5,6,7,8,9)        #=> 2003-04-05 06:07:08 UTC
    p Time.local(2013,10,28,16,27,48)  #=> 2013-10-28 16:27:48 +0900

Implementation:

I use `strftime(3)` because UTC offset can be added and program size become
smaller than the other implementations (using `sprintf(3)`, self conversion
etc) in my environment.