adding digital_ociean

Hi, kenn

It support [Digital Ocean](https://www.digitalocean.com) API.

FYI, [The Digital Ocean API](https://www.digitalocean.com/api) seems like beta version.
You must create your ssh-key on your hand 'cos API has many incompleted commands.

If you like it, please merge it.

Thanks.
Keisuke Kawahara