Merge from vendor branch TNF:

Merge from vendor branch TNF:
Importing devel/ruby-activemodel 3.0.0.

Active Model provides a known set of interfaces for usage in model classes.
They allow for Action Pack helpers to interact with non-ActiveRecord models,
for example. Active Model also helps building custom ORMs for use outside of
the Rails framework.

(This is part of Ruby on Rails 3.)