Enable send/receive#6

This worked for me. On the receiving side I had to: ``` sudo zfs allow $(whoami) receive,create,mount tank ``` On the sending side I had to ``` sudo zfs allow -u $(whoami) send,hold tank ``` then this worked from an un-elevated user: ``` local_dataset.receive(remote_newest_snapshot.send(remote_common_snapshot)) ``` where the two snapshots were from a zfslib.Connection() to one server and the dataset was a zfslib.Connection to another. When there were errors, I saw them and the program I was running exited. Had I chosen to trap more exceptions in my own code, I would have been able to decide not to exit.