Add rated_at timestamp when adding ratings.
Support collected_at timestamp for adding to collection.
Merge branch 'timestamps-for-sync' into api-v2
Add entity id builders, remove args constructors.

- Not providing a no-args constructor is potentially unsafe (see http://stackoverflow.com/a/18645370/1000543). Adding them in addition defeats the purpose of ensuring all required args, so reverting.
Use different show to pass add season rating test.