0.1.3 (2013-12-28)
  - Implement ArrayBranch and HashBranch.  These bundle ArrayComposite and
    HashComposite up with the Movable and ParentTracker mixins, for
    convenience.

0.1.2 (2013-12-27)
  - Implement and specify #get_child.

0.1.1 (2013-12-27)
  - Avoid a function name clash between #remove_parent on a child and
    #remove_parent on a parent.  The latter is now #remove_parent_of.
    #assign_parent is now #assign_parent_to, to match.

0.1.0 (2013-12-26)
  - Initial version.