Squashed commit of the following:

commit 8bc9a593346acdda696395546dc7a36dd37f2592
Author: Satoshi OKANO <satokano@gmail.com>
Date:   Tue Apr 8 03:29:55 2014 +0900

    delete REXML

commit f8dd8aeb9beda0772ed0cf45fd18d6c73d8da822
Author: Satoshi OKANO <satokano@gmail.com>
Date:   Mon Apr 7 04:19:29 2014 +0900

    rewrite rexml -> nokogiri