Progress bar for file uploads

This is a PR for #262 .
ProgressBar now extends from UploadProgressBar (wicket-extensions) to re-use its UploadStatusResource.
An example of file upload with progress is added to the extensions page (Jasny File Upload, demo 1).

I am not sure whether I didn't break some use case of the current ProgressBar with the change to extend from UploadProgressBar. I think it should be fine but since there are no other examples I am not 100% sure.

The only thing that is known to be broken at the moment is de.agilecoders.wicket.core.markup.html.bootstrap.components.ProgressBar.Type (http://getbootstrap.com/components/#progress-alternatives). It will be fixed with Wicket 6.13 release that will provide template method for creating the component for the progress bar.

@l0rdn1kk0n @dmbeer Please review/test.