A Rack JSONP middleware

This is a customized implementation of a JSONP middleware.

The main difference with the rest of them is that this one will add JSONP support to any of your JSON calls but only when the extension name '.jsonp' is present.

Since 'callback' is a really generic parameter name if someone wants to get a JSONP response they must request it explicitly.

Btw, don't forget to give a try to J50Nπ (a pure JS JSONP helper), they make a lovely couple together :P