Store `path` property on each node

[sloria]

Besides the fact that this will eventually be needed for certain OSF features (e.g. pointers, S3 addon), this would be a useful feature to have

[sloria]

To be more clear:

var root = new HGrid.Tree();
var tree = new HGrid.Tree({name: 'My Docs', kind: 'folder'});
var leaf = new HGrid.Leaf({name: 'foo.txt', kind: 'item'});
root.add(tree);
tree.add(leaf);
console.log(leaf.path);
// => ["My Docs", "foo.txt"]

path might also be an Array of IDs instead of names.