Implement API non-RESTful format, JSONP, CORS

Fixes #53.
Long-term, yes error objects:

https://github.com/zoomhub/zoomhub/blob/master/API.md#v2-wip

(I don't think TODOs are necessary in the code for long-term things when we have roadmap documentation.)

For now, yes plaintext error messages by design, for Zoom.it v1 API compatibility:

https://github.com/zoomhub/zoomhub/blob/master/API.md#routes

http://old.api.zoom.it/v1/content?url=
http://old.api.zoom.it/v1/content?url=&format=json