Git commits in Moodle 2.3.1 by ISHIKAWA Takayuki

commit d0d407f0bb7a3996e551fb9aa4a653a5c9bd28f9
Author: ISHIKAWA Takayuki <rechka_osaka@yahoo.co.jp>
Date:   Thu, 14 Jun 2012 20:29:33 +0800
Tag:    v2.3.1

MDL-33757 gradebook: Adding support to sort userlist by email address