Fix connection issues in 0.11.0 release

Trying to address issue #632, I believe this commit https://github.com/mranney/node_redis/commit/9c68a286b1b0be913bbbca112ea4d241eda96d91 broke the ability to connect to a redis instance listening on a unix socket in this fashion:
```
var redis = require("redis"),
     redisClient = redis.createClient("/tmp/redisTest.sock");
```
as well as being able to connect to redis when passing in the cnxOptions directly as:

```
var config = {
   port: 6379, 
   host: '127.0.0.1',
   options: {}
};

var redis = require("redis"),
     redisClient = redis.createClient(config);
```
my pull request attempts to check if you are sending a unix path into port_arg and set the cnxOptions correctly to net.createConnection

this should fix some, if not all of the connection regressions described in #632