replacing dirty's changed_attributes with original_values

I was curious what the code would look like without all the tracking of `@changed_attributes`, but instead just storing the `@original_values` for active model/record attributes.

It only stores the original_values upon `attribute_read` / `attribute_write`.

I got it close. The last commit (that removes `@changed_attribtues`) breaks the test that detects when a numeric field is changing from `0` to 'foo', along with one of the date tests.

requesting comments