Backtracking some python 2.5 incompatible changes

Just a minor fix to doctest_driver.py

Python 2.5 does not accept @property.setter decorator, so I changed them to uglier code

This reverts commit 1c841d33.

Replace standard doctest nose plugin only if it doesn't support --doctest-options

Mostly doctest issues

The only thing that was used from epydoc was the TerminalController, so I simply copied it from epydoc.util to the doctest driver

Test running improvements

Trees are not lists

Properties in @decorator style, etc.

The following properties were changed to methods:
- ParentedTree: parent, parent_index, left_sibling, right_sibling, treeposition, root
- MultiParentedTree: parents, left_siblings, right_siblings, root

Note that this might break existing code that uses parented trees.

Cleanup, new regression tests and properties in tree.py

E.g., changing obj.__class__ to type(obj), and better names for variables

Remove `__new__` from Tree classes

Having both __new__ and __init__ in the same class that also has subclasses can lead to problems, and easily becomes very unreadable.  I removed the __new__ method and put its functionality into __init__ instead.  I also updated the ValueError's for the Immutable classes.

Allow steps in slices for all kinds of Trees

Fixes for issues #196 and #198