Droplets can be created with user_data and private_networking

* Add user_data attribute to Droplet resource and mapping
* Add private_networking attribute to Droplet resource and mapping

I think these features are awesome so I wanted to be able to access them from DropletKit.

Also, there are appears to be some failing tests on master. A lot of tests are expecting a `XXXResource` but are getting `PaginatedResource` instead. I didn't have a whole lot of time to investigate unfortunately.