Improve the support of unix socket 

I noted the [issue #204](https://github.com/mranney/node_redis/issues/204). But, I got an error using redis.createClient('/tmp/redis.sock'). Not sure if this is because the interface of net.createConnection() changed. 

Anyway, IMHO, unix socket is a useful feature that need to be there. It provides better isolation, faster performance. And yet this is not that much of work. 

I removed client.host and client.port, because they do not fit for unix socket.
And add client.address and client.connectionOption. The former is better to reference addres and the later to reference the way to create connection. 

I also added new test case UNIX_SOCKET and updated the testcase CHECK_LIST. This pull request pass all test cases. 

Finally, README is updated. 
Oops, typo here that breaks tests on older versions of Redis. I'll take care of this before I release.