JSON output

Hello, I've roughed in a patch to make rest_server output data in JSON, by adding an argument 'format=json' to the URL.

This is somewhat hardcoded, but I think this makes a lot of sense, since REST doesn't means the output should be in XML (well, everyone know this :).

What do you think?

PS: I know about json_server, but in my case, I just need the same GET-based interface rest_server provides, and the output in JSON.