Dirty
=====

A newly instantiated object is unchanged:
  person = Person.find_by_name('uncle bob')
  person.changed?       # => false

Change the name:
  person.name = 'Bob'
  person.changed?       # => true
  person.name_changed?  # => true
  person.name_was       # => 'uncle bob'
  person.name_change    # => ['uncle bob', 'Bob']
  person.name = 'Bill'
  person.name_change    # => ['uncle bob', 'Bill']

Save the changes:
  person.save
  person.changed?       # => false
  person.name_changed?  # => false

Assigning the same value leaves the attribute unchanged:
  person.name = 'Bill'
  person.name_changed?  # => false
  person.name_change    # => nil

Which attributes have changed?
  person.name = 'bob'
  person.changed        # => ['name']
  person.changes        # => { 'name' => ['Bill', 'bob'] }