Add timestamps fields to spree_assets

Initial motivation here was make assets_spec green. As of Rails 4.0.4
touching an object without timestamps won't touch its associations. Plus
timestamps are usually handy so no reason for not having them in
spree_assets too

See https://github.com/rails/rails/commit/73ba2c14cd7d7dfb2d132b18c47ade995401736f
Use stub_model rathen than mock_model

Make rails 4.0.4 build happy