fix bug with getting cell address if cell column will more then 26

you can test previous version with this code 

```javascript

var excelbuilder = require('msexcel-builder');

// Create a new workbook file in current working-path
var workbook = excelbuilder.createWorkbook('./', 'template.xlsx')

// Create a new worksheet with 10 columns and 12 rows
var sheet1 = workbook.createSheet('sheet1', 201, 30);
sheet1.merge({col:1, row: 1},{col: 200, row: 1 });
// Save it
workbook.save(function(ok){
	if (!ok) 
		workbook.cancel();
	else
		console.log('congratulations, your workbook created');
});

````

this code will generate xml file but this file will be corrupted.

My changes will fix this issue. 
Test file just in case.