過去一週間分の日付文字列生成
((Date.today - 1.weeks) ... Date.today).map{|e| e.strftime("%Y-%m-%d")} => ["2012-07-18", "2012-07-19", "2012-07-20", "2012-07-21", "2012-07-22", "2012-07-23", "2012-07-24"]
((Date.today - 1.weeks) ... Date.today).map{|e| e.strftime("%Y-%m-%d")} => ["2012-07-18", "2012-07-19", "2012-07-20", "2012-07-21", "2012-07-22", "2012-07-23", "2012-07-24"]