add support to uploading to a resource

Add support for the following:

• Upload to a resource in an asynchronous non page blocking request
• Add an optional way to block the user from leaving the page while the upload is happening
• Ways to cancel the upload
• Adapt the upload progress bar to work with this new "component" and improve its code as in some corner cases it is producing client side errors (I created an issue for that some time ago).
• Maybe useful too: create a web socket based progress bar, as the upload progress bar now works pulling the server every second.
• Also to add an example to wicket-examples that uses a smart JS uploader, like in the blog (https://github.com/martin-g/blogs/blob/master/file-upload/). This way you will verify that the new APIs are easily extendable.