Hetznercloud

* Adds new server type constants
* Adds new image constants
* Parses the IP field from the created floating IP API calls
* Fixes the cloud-init documentation.

Allows new datacenters to be created in any datacenter in a selected location. See pull request 16.

Adds the new image type constants. See pull request 17.

Fixes an issue with the 8GB server constants having the wrong type. See issue 15

* Updated the README to show supported Python version and this changelog section.
* Fixed an issue with status codes returned from server actions.
* Fixed an issue where the body is null when sending a POST request.
* Added the new Helsinki DC as a constant.
* Added a check to the shared API requestor that checks for the 429 rate limited status code and returns a suitable exception.
* Added an alias for datacenters, as my Britishness got the best of me when creating this library.