code: def total_return(self):
          
          pxend = self.close
          pxstart = pxend.shift(1).bfill()
          return (1. + (pxend - pxstart + self.dvds.fillna(0)) / pxstart).cumprod() - 1
docs: http://en.wikipedia.org/wiki/Total_shareholder_return - mimics bloomberg total return