Add content_type and size errors to base attribute

This propagates attachment size and content_type validations to the main 
attachment attribute, to make them behave the same as presence validations.
This allows paperclip to play more nicely with simple_form and other form
abstraction gems.

This commit includes a behaviorial change. If you emit all errors on a model
instance you will see some failed validations twice: once for the base
attachment attribute and again for the failed presence, content_type or size
attachment attributes.