Added CSV Export

I extended the jqGridExport method to allow CSV exporting. 

You have 2 extra parameters:

```javascript
hideColumns: [];
```

You will use it to choose columns that wont be exported. You have to use colModel.name.

```javascript
separator: ";";
```

Choose the separator.

You can use it in this way:

```javascript
var csv = $("#myGrid").jqGridExport({
                exptype: "csvstring",
                hideColumns: ["Cod"],
                separator: ";"
            });
```