#1048: Create a JSONP writer

For phpDocumentor to create templates that consume content using XHR requests we
actually need to do JSONP requests to retrieve content. This is because docs
made with phpDocumentor are often read offline without the use of a web server.

The absence of a web browser has the disadvantage that XHR requests are not
possible with the single exception of JSONP requests because those are not
hindered by cross domain request restrictions.
You know, the more common thing here would have been putting the blocks together with `elseif`.  Since I didn't even consider saying that (even though it was my first thought), I think I've taken @rdohms Object Calisthenics to heart.  Granted, now that I realize this, I bet those probably say "don't use `continue` to cheat" :-P