New features:
* Support for retrieving reverse PTRs.
* Support for subnet-ranges.
* Add logging (14).
Fixes:
* Hide internal methods from the public API.
* Fix Python 3 compatibility.
* Fix for creating admin accounts with Hetzner's new login site.
* Fix `__repr__`/`__str__` issue with some exceptions (23).
* Fix login for `RobotWebInterface`
Changes for the `hetznerctl` utility:
* show: Show subnets
* show: Show reverse PTRs
* New `rdns` subcommand for getting/setting/removing reverse-PTRs.
* Use `argparse` instead of `optparse`.
* Add command for managing admin accounts.
* New `--debug` flag for printing debugging information.