Add method to retrieve "active" bundle.

Add a helper method to the `Bundles` class, retrieving the "active" bundle. This is determined by checking the `active` field on the bundle object. Currently, the this can only be done by retrieving the full bundle list and then filtering for the match.

```python
client = connect.Client()
content = client.content.get(guid)
bundle = content.bundles.active()
```