Add ajax dataFilter to filter the returned data from jQuery.ajax for the load_on_demand feature

[cheton]

Since the returned result of load_on_demand from backend server may not in the form of jqtree's data format, it will be great if a dataFilter function is provided as tree option.

Example:

$.tree({
    dataUrl: function(node) { ... },
    dataFilter: function(data) {
        // process the load_on_demand data
        return data;
    }
});

In the tree.jquery.js

$.ajax({
    dataFilter: _this.options.dataFilter
});

[mbraak]

Yes. I think this is a good idea. I will add this option.

[mbraak]

The dataFilter option is now added to the dev branch.