Traverse over nested objects

This change is to make slickgrid able to traverse over nested objetcts. For example we could define the next columns:

var columns = [
    { id: "antal", name: "Antal", field: "num", width:  10},
        { id: "namn", name: "Artikelnamn", field: "article.name", width: 50},
        { id: "pris", name: "Totalpris", field: "article.price", width: 50}
];