Adding Unix domain support to the celery transport

As discussed there :
https://github.com/celery/celery/issues/1283
https://github.com/piquadrat/celery-redis-unixsocket/issues/3

This is the upstream merge of celery-redis-unixsocket.
It allows you to connect to redis over a Unix socket:

    from kombu import Connection
    conn = Connection('redis+socket:///tmp/redis.sock')

Commit includes:
 - Unix socket support
 - Connection.as_uri() update to correctly return `redis+socket:///tmp/redis.sock`
 - a test
 - documentation update

@ask could you review and test my code ? Thanks !