RabbitMQ extensions support


```
conn = Connection(host="host", userid="user",
                  password="password", virtual_host="vhost",
                  client_properties={'capabilities': {'authentication_failure_close': True}})

conn.connect()
```

PR's https://github.com/celery/librabbitmq/pull/45 and https://github.com/celery/librabbitmq/pull/44 are commits that are also part of this PR but can be merged separately.

Previous implementation (https://github.com/celery/librabbitmq/pull/46) works fine but now we expose client_properties as the API instead of capabilities and also enable support within the librabbitmq Python __init__.py file.  

This current approach also allows the connection parameters to be initialized as Kombu Connection objects too.