Partial responses.

GET output can be filtered with `fields=` in URI. Replaced underscore
dependency with lodash (primarily to get merge()).
Respond to HEAD requests.

All `GET` routes how respond properly to `HEAD` requests, rather than
sending a 405. Partial response now doesn't screw up error messages. We
now send a full set of headers back, including `API-Version`, which is
1.0.0.
More useful status output.
Update README.md