allow a render function via render attribute for each node

Hi Vitali,

instead of .html i changed this to .render for EACH node.

Function argument is the node object so a render-function would look like this:
<pre>
...
  render: function(node) {
    return 'Default';
  }
...
</pre>

Return value also goes into .text property of the node. Another point is, that for the root-nodes, there will be no   span.../span (I needed this to probably display input fields in a root-node).