INode.get_output allows receiving stdout of remotely run commands.
Address review comment: Less obscure tests.
Address review comment: More info in exceptions.
Merge remote-tracking branch 'refs/remotes/origin/master' into ipc-stdout-222
Merge pull request #223 from ClusterHQ/ipc-stdout-222

INode.get_output allows receiving stdout of remotely run commands.

Fixes #222.