Move ActiveModel::AttributeMethods#attribute_methods_generated? to ActiveRecord

In order to add ActiveModel::Dirty to models, people should use AttributeMethods. Everything is cool with AR-like models, but consider model like this:

  class Article < Model
    property :title
    property :body
    property :published_at, Time
  end

Currently we should use AttributeMethods#define_attribute_methods([:title, :body, :published_at]). In this DataMapper-like model it's impossible to determine all properties or find a nice workaround. Also, we can't call #define_attribute_methods multiple times, because it does nothing if #attribute_methods_generated? is positive.

My patch moves all this #attribute_methods_generated? tracking to ActiveRecord. Also #define_attribute_method added. All AR tests passes, Am test modified. Yeah, there is no backward compatibility, but it's pretty simple to override AM behavior just like it done for AR (see patch for details).