Support destination directory in the zipfile

I want to add `dest` option for setting destination directory in the zip file

Usage:

```
gulp.src(["apps/app-main/**", "apps/app-chat/**", "apps/app-xxx/**", "modules/**", "package.json", "index.html"])
    .pipe(zip('archive-141203.zip', {dest: "webapp"}))
    .pipe(gulp.dest('release'));
```


output zipfile structure

```
archive-141203.zip/
    webapp/apps/app-main/
    webapp/apps/app-chat/
    webapp/apps/app-xxx/
    webapp/modules/
    webapp/package.json
    webapp/index.html
```

When no `options.dest` parameters , directories, zip files is like:

```
archive-141203.zip/
    apps/app-main/
    apps/app-chat/
    apps/app-xxx/
    modules/
    package.json
    index.html
```

`options.dest` parameter allows `gulp.src` each item , output to the same directory , instead of the root zipfile

In one of our projects , we need to be labeled as a multiple apps zip package , if the compressed file is not the root directory , it will be very messy after decompression . 

So I need to add a parameter `options.dest`

thanks!