Added cellAtributes on jqGrid column renderer fixes #107

This adds the following valid `cellCttributes` to the column renderer:
- sorttype
- summaryType
- summaryTpl

Though valid strings are defined their validation is not implemented yet.

These `callattr` quickly allow the user to use jqgrid's summary feature on the `footerrow`.
See *Grouping* -> *Summary Footers* here: http://trirand.com/blog/jqgrid/jqgrid.html

Still `groupingView` parameters have to be added to the jqgrid `tempalte`e.g.:
```javascript```
 grouping:true, groupingView : { groupField : ['name'], groupSummary : [true], groupColumnShow : [true], groupText : ['<b>{0}</b>'], groupCollapse : false, groupOrder: ['asc'] },
```

A quick fix to issue https://github.com/ThaDafinser/ZfcDatagrid/issues/107 .