added natpf command for easy NAT port-forwarding settings.

There are many times to set NAT port-forwarding settings. 

For example, http://docs.docker.io/en/latest/examples/nodejs_web_app/ 
I'd like to see that application from locale browser.

So, I added VBoxManage's controlvm command's wrapper.

```
$ boot2docker natpf "node,tcp,127.0.0.1,49160,,49160"
$ boot2docker natpf delete node
```