Implement a new validator: `UnchangedValidator`

This ensures that the specified model attributes have not changed. For example:

    class Person < ActiveRecord::Base
      validates_unchanged :first_name
    end

    p = Person.first
    =>  #<Person id: 1, first_name: 'Dave'>

    p.first_name = 'Bob'
    p.valid?
    => false

    p.first_name = 'Dave'
    p.valid?
    => true

This is most useful when there are attributes on a model that should only be
set once, or at least only rarely.