FR: Cloud Bigtable Client class should be a Context Manager

It would be great if I could say:

```
with bigtable.Client() as client:
    cluster = client.cluster('us-central1-c', 'my-cluster')
    # ...
```

and the Client class would call start and stop for me. I believe we could make  `__enter__` call `start` and `__exit__` call `stop`.
