LONDON (Reuters) - European shares tumbled again on Monday as a profit warning from online fashion retailer ASOS sent retail stocks spiralling down as investors fretted that consumers are failing to deliver the traditional pre-Christmas spending boost to markets.Euro zone stocks were down 0.5 percent while Germany's DAX fell 0.2 percent and Britain's FTSE 100 lost 0.3 percent. ASOS shares plunged 36 percent after the British online fashion retailer cut its forecasts, saying November was..