introduce gulp-qunit.finished event

In order to trigger other tasks, depending on the test-results, I have introduced a `gulp-qunit.finished` event which holds the overall result status.

In my case I run a S3 deployment when all tests passed:

```
gulp.task('test', function() {
    return gulp.src('tests/TestRunner.html')
        .pipe(qunit())
        .on('gulp-qunit.finished', function(result) {
            if (result.passed) {
                aws = JSON.parse(fs.readFileSync('./aws.json'));
                gulp.src('./dist/**')
                    .pipe(s3(aws));
            }
        });
});
```