JSONP parameter in Action API

[dread]

Action API needs JSONP support - be able to return responses encapsulated in a function of a supplied name. This is important for remote sites running javascript to interact with a CKAN site.

Specifying the callback parameter is the way we've achieved JSONP with the RESTful and Search APIs. It should work like this:

curl http://test.ckan.net/api/action/package_show?callback=jsoncallback -d '{"id": "fd788e57-dce4-481c-832d-497235bf9f78"}'

Or maybe the callback should be specified in the payload in the context or data_dict?

(My understanding is that CORS is similar - when more browsers support it, can we drop JSONP?)

[dread]

This can be fixed with one line of code, so I'm doing it for release 1.5.