Add __eq__ and __ne__ methods to Tree.

Add __eq__ and __ne__ methods to Tree.

This fixes a bug introduced by
  revision-id: wking@drexel.edu-20091205034412-8apqxq8zqim48tf7
  committer: W. Trevor King <wking@drexel.edu>
  timestamp: Fri 2009-12-04 22:44:12 -0500
  message:
    Use __cmp__ instead of __eq__ for Tree comparison.

When I made that commit, I'd forgotten that Tree inherits an __eq__
method from list, so it won't fall back to the __cmp__ method to
determine equality.  The new __eq__ and __ne__ methods use __cmp__
internally, so further subclasses (e.g. Comment) only need to override
__cmp__.  Of course, list also defines __ge__, __gt__, __le__, __lt__,
... which I don't bother with, so stay away from TreeA > TreeB and the
like.