Tree can read itself

I will use this in some tests around reports. The tree representation is in ui_navigate style.

Usage eg.:
```python
>>> accordion.tree("Policies").read_contents()
[(u'All Policies',
  [(u'Compliance Policies',
    [u'Host Compliance Policies', u'Vm Compliance Policies']),
   (u'Control Policies',
    [u'Host Control Policies',
     (u'Vm Control Policies',
      [(u'test', [(u'VM Power Off', [u'Convert to Template'])])])])])] #I hate these
```