INode.get_output allows receiving stdout of remotely run commands (fixes 222)

Fixes #222.
Something like 
```python
try:
    return check_output(
          self.initial_command_arguments + remote_command)
except CalledProcessError as e:
    raise IOError("Bad exit", remote_command, e.returncode, e.output)
```
would make debugging failures easier, even if it isn't a great interface. (I've done this a couple of times with the analogous code in `f.v.f.zfs`).