move ant's build directory from "bin" to "build"

commit 25635373d4f8d1779f3d9d9a19a05c9f3a5dc67d
Author: Masahiro Ide <imasahiro9@gmail.com>
Date:   Wed Oct 9 11:00:34 2013 +0900

    update build.xml and Makefile

commit 5f621465a52c0eeaadc216af73fe7a39219e9140
Author: Masahiro Ide <imasahiro9@gmail.com>
Date:   Tue Oct 8 14:40:16 2013 +0900

    update build.xml