added a JsonpHandler

fixes #263

* [x] rebase
* [x] decide on how to recognize jsonp requests

The issue with recognizing jsonp requests is the following:
If users will just send ``application/javascript`` in their Accept headers, we will not know if they want ``js`` or ``jsonp`` since ``application/javascript`` will always be mapped back to just ``js``. There are four solutions:

1) use ``application/javascript+jsonp`` or some similar mime type that is mapped exclusively to ``jsonp``
2) simply assume that ``js`` means ``jsonp`` when the ``jsonp`` handler is active
3) require that controllers set ``jsonp`` explicitly as the format ``$view->setFormat('jsonp')``
4) make the mime type configurable (currently implemented)

I should not that using 1) of course does not prevent someone form using 3). However in that case the Accept header would become irrelevant and also the content would still be served with ``Content-Type`` set as ``application/javascript+jsonp``