Add asset uploader pipeline support

[jeremydw]

One internal project I've worked on has an asset pipeline that automatically uploads files from Google Drive to Google Cloud Storage, keyed by folder title and folder modification time.

We should:

• Add official support for this type of pipeline to the extension.
• Remove dependency on objectcache.json for files uploaded by the pipeline (auto-uploaded assets generate objectcache data on the server rather than the client, so the cache data can be pulled offline and stored locally in cache files keyed by folder title).
• Add support for locale groups when working with localized assets (so [email protected] will work, where foo is the name of a Grow locale group defined in podspec.yaml)