Provides a `done` callback for `gulp.task`

This commit adds the ability for a done callback to be executed from
`gulp.task.` It can be used like so:

```js
gulp.task('deploy', function (done) {
    return gulp.src('dist/**', {read: false})
        .pipe(s3(aws, options, done));
});
```

This ensures that this `deploy` task can be chained correctly.