Add rates_updated_at attribute containing the date set in the XML file

Saves the modification date of the rates.

Can be used for a cache expiry implementation like so:

	@bank = EuCentralBank.new

	cache = "tmp/ecb_exchange_rates.xml"

	begin
	  @bank.update_rates(cache)
	rescue
	end

	if !@bank.rates_updated_at || @bank.rates_updated_at < Time.now - 1.days
	  @bank.update_rates
	  @bank.save_rates(cache)
	end