Allow AMQP extensions to be passed when connecting to RMQ broker.

The Kombu library should be modified to support the extensions (https://www.rabbitmq.com/extensions.html).  For now, the librabbitmq extension can be invoked as follows:

```
conn = Connection(host="host", userid="user",
                  password="password", virtual_host="vhost",
                  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.