Hi,

The pull request changes are for the following reasons.

1. I am currently writing a FileTreeView which has the interface

   public FileTreeView(Context context, File file) {
       super(context);
       TreeNode root = TreeNode.root();
       setUpNode(root, file);
       setRoot(root);
   }

   This requires creating a root node before initialising the superclass AndroidTreeView.

2. mRoot usually needs to be visited in subclasses when writing a custom traverse method. I changed it to protected.

3. I have the need to access the nodeContainer from others classes.
   For example, I want to set the background of a single row to a certain colour.

this.highlightedNode.getViewHolder().getView().setBackgroundColor(color);;

This would colour all the children nodes.

Instead, I would like to do something like this:

view = (TreeNodeWrapperView) this.highlightedNode.getViewHolder().getView();
view.getNodeContainer().setBackgroundColor(color);