Update Go API client to be closer to API spec

Allow retrieving features for a region. The features will
be available in the `Features` member of your `Region` struct.


Items allowed by the API were missing from the client. The new
options are: backups, ipv6, private_networking, and user_data.

See [our announcement](https://www.digitalocean.com/community/tutorials/an-introduction-to-droplet-metadata)
for more detail on the usage of user_data.


Fixed a condition where determining the current page of results would panic.