Add s3upload:form-data event.

This allows us to add extra headers to the form data,
something very useful when we want to provide Metadata.

Example:

    app.run(function($rootScope) {
        $rootScope.$on('s3upload:form-data', function(event, fd) {
            // set the cache-control header, fixed string
            fd.append('Cache-Control', 'max-age=31104000, public, must-revalidate, proxy-revalidate');

            // set the expires header, one year from now
            var now = new Date();
            var oneYear = new Date();
            oneYear.setYear(now.getFullYear() + 1);
            fd.append('Expires', oneYear.toUTCString());
        });
    });