--- sort
--- sort!
--- sort {|a, b| ... }
--- sort! {|a, b| ... }

    Ƥ򥽡Ȥޤ֥åȤȤ˸ƤӽФ줿ˤ
    ֥å 2 Ϳɾη̤Ӥޤ
    ֥åʤˤ <=> 黻ҤȤäӤޤ
    sort ϥȤ줿֤sort! 
    self ˲Ūѹޤ

    ruby 1.7 feature:
    sort! ϡС 1.6 ˤǤο 2 꾮ˤ
    nil ֤ƤޤС 1.7 ǤϾ self 
    ֤ޤ

