initial commit for multi machine control

Trying out something (and so far it's working quite well!):
- combine a bunch of machines into a proxy object using '+' operator:
```python
    m = local +  SSHMachine(host1) + SSHMachine(host2)
```
- then invoke commands in parallel:
```python
    r1, r2, r3 = m["du"]("/tmp")
```

You can control a whole cluster like this! :)