use cellIndex to get sortIndex from th

Using an array of objects as a data source I found
```javascript
var sortIndex = parseInt($(event.target).attr("index"), 10)
```
is `-1`, so `var sortKeyField = this.fields[-1]` is `undefined` and causes
`TypeError: Cannot read property 'fn' of undefined` on line 178 of reactive_table.js when `sortKeyField.fn` is referenced.

Any reason not to use `cellIndex`? Works well for me :)