Added display function in grid column object.

I have just add a display function in grid column object. Now developer
can manipulate the cell content before generating.

{width: 30, header: "Amount", display : function(node){
// node.attr() for attribute
// node.data() for metadata
var md = node.data();
var discount =  (md.price / 100 ) * md.discountInPercent;
return md.price - discount;
}}