Implement scp functionality (#149)

* Redefine SCP interface

* Implement Machine.scp_to and Machine.scp_from

* Implement Unit.scp_to and Unit.scp_from

* Fix Unit.machine to better handle missing machines

* Use os.path.expanduser for scp key path

* Use local-cloud address if public address is not found

* Rename Machine.public_address to Machine.dns_name to match CLI

* fix scp tests not waiting long enough