Return a file buffer containing the resulting xls

        :param obj f_buf: A file buffer supporting the write and seek
        methods

def save_book(self, f_buf=None):
        """
        Return a file buffer containing the resulting xls

        :param obj f_buf: A file buffer supporting the write and seek
        methods
        """
        if f_buf is None:
            f_buf = StringIO.StringIO()
        f_buf.write(openpyxl.writer.excel.save_virtual_workbook(self.book))
        f_buf.seek(0)
        return f_buf