Handle empty rows

The problem is that Excel doesn't generate `<row>` tags for empty rows, but simple_xlsx_reader expects the next row to include cells `rownum + 1`.

I took a different approach here, by iterating through xml row/cels nodes, instead of guessing row and cell numbers. This is similar to how rubyXL handles it.

It also now has one XPath query to get all columns for a row.