simple jsonp support

Lots of people use node-static to setup a simple http server locally. I think some of them would like simple jsonp supported.

Here is the rule: 

Any request with query like `?_callback=abc` will be treated as a jsonp request.
For example, requesting a json file named 'test' with path `/test?_callback=abc`:

    {
        "foo": "bar"
    }

will output like this:
    
    abc({
        "foo": "bar"
    });

Test cases and docs are included. Hope it will help :p