How to export the base box?

Thanks for your work! It help me to build the base box in windows7. But I couldn't find how to export the base box after build, validate it.

```
$ bundle exec veewee vbox build 'centos63'
$ bundle exec veewee vbox validate 'centos63'
```

what's next? there is no export parameter if I run this:
    $ bundle exec vagrant basebox export 'centos63'
