Add IPv6 support to kazoo

Kazoo lacks IPv6 support which makes it difficult to use it in our company, where zookeeper lives on IPv6 addresses only. Looks like it's a right time to teach kazoo some IPv6.

UPDATE:
Almost forgot to describe an interface:
```python
zk = KazooClient(hosts='[fe80::1%lo0]:2181, 127.0.0.1:2181, etc')
```