Migrate API endpoints and CLI client to JSON Siren 

These patches change the output of all `/api/collections` endpoints to JSON [Siren](https://github.com/kevinswiber/siren), and add helper methods to treat object collections more REST-fully. The `/api/commands/...` endpoints are no longer advertised but are still available, though they are generated through the new helpers instead of the `command` helper previously in `app.rb`.

In addition, this patch set updates the CLI client to parse and make use of JSON Siren (like field names for command-line flags) for lists, details, and actions.