add support for PowerOn

as specified in https://developers.digitalocean.com/v2/#power-on-a-droplet
Merge branch 'nitrous-io-add-poweron'
Fix example in readme

- client.Droplet doesn't exist
- Image takes a string, not an int
- Ubuntu image ID seems to have changed
Merge pull request #7 from bfirsh/fix-example-in-readme

Fix example in readme
If pages are empty, assume we are on last
Add all options to droplet create request

Added:

backups, ipv6, private_networking, user_data
Retrieve features for a region
Merge pull request #8 from digitalocean/userdata

Update Go API client to be closer to API spec