Make AM::Dirty less dirty to plugin into AR or other library

Added new API methods #reset_changes! and #changed_applied!
to control changes state. Previsously you needed to update internal
instance variables, but now API methods is available.


``` ruby
    class Person
      include ActiveModel::Dirty

      define_attribute_methods :name

      # Old way:

      def save
        # do some work
        @previously_changed = changes
        @changed_attributes = nil
      end

      def reload
        # do some work
        @previously_changed = nil
        @changed_attributes = nil
      end

      # New way:

      def save
        # do some work
        changes_applied!
      end

      def reload
        # do some work
        reset_changes!
      end
    end