Updates to Dataset row/column access methods

* Remove method, prefix and suffix from dataset formatter
* Include index as second argument of appendColumn and appendRow
* Append/Insert add empty rows when nothing is returned from the iterator
* Update only modifies data when values are returned from iterator
* Added a new pair of methods: formatRow + formatColumn, for iterating over each cell in a given range, with the current value as an argument. eg:

```
var ds = new Keen.Dataset();
ds.formatRow(0, function(cell, index){
  return String(cell).toUpperCase();
});
```