New feature: add CORS support

When a developer is working on a website/webapp that is accessing miniProxy using AJAX, the developer might have issues with CORS (Cross-origin resource sharing) that will prevent him/her from getting
the proxified data. For example if the webapp and the service are not on the same domain, the browser will block the request.
I added a way to easily enable CORS support. All the developer needs to do is edit config.php and set $forceCORS to true (the default is false, which will act exactly as before).

Using this will make it a lot easier to use the service through AJAX.