Added rollback method to ActiveModel::Dirty

Maybe I have missed something but looks like rollback changes should be presented here
Sorry, it should have the `# :doc:` flag added. Private methods are not documented by default, this flag will make sure it is documented. You should add it on the same line as the method definition like so:

```ruby
def rollback_changes # :doc:
```

http://rdoc.sourceforge.net/doc/