Add key retrieval example using fabric in docs

It would be nice to show in the docs how fabric could utilize ghost to read keys:

```python
import ghost
from fabric import run, env

stash = ghost.Stash(ghost.Storage(...), ...)
env.key = stash.get('ssh_key')['value']['key']
env.host_string = stash.get('ssh_key')['value']['conn']
...
```