Provide more useful Ports data in Client.containers() output

When calling `Client.containers()` the return value for `Ports` is a string that looks like:

```
u'Ports': u'5000->5000, 6000->6000'
```

For programmatic consumption, the value would be much more useful as a sequence of `(public_port, private_port)` tuples, like:

```
u'Ports': ((5000, 5000), (6000, 6000))
```

Thoughts?
