Implement create_backup

Adds function to tell the UniFi controller to prepare a fresh
.unf backup archive that can then be downloaded. The obtained
file can be used to restore a UniFi controller via GUI or at
initial setup.

JSON command obtained via publicly available API shell sample code
for UniFi 2.4.4 (http://www.ubnt.com/downloads/unifi/2.4.4/unifi_sh_api)

Apparently no authentication is required for the file, once it has been
created by the server. Tested with basic urllib.urlretrieve and plain wget.