Fix: acts_as_forest can be called more than once

acts_as_forest defined a private method as the last thing it did.
If acts_as_forest was called more than once, whether called twice
in the same model, called in two different models, or called
twice via Rails reloading, the redefinition of find_forest and
friends would be defined as private.

Fix by separating class methods into module that acts_as_forest
extends the target classes with instead of writing the methods
directly. In particular, the old way failed because the methods
were defined on the ClassMethods module instead of the class that
called acts_as_forest
Bump version and add changelog