improved connz with subscriptions stats and list of subscriptions by client

I just added some extra info to stats page:

1. Added subscription stats 
2. Added a list of subjects for each client
3. Renamed `subscriptions` to `num_subscriptions` (this is breaking backward compatibility)

```json
{
  "num_connections": 1,
  "stats": {
    "num_subscriptions": 1,
    "num_cache": 0,
    "num_inserts": 1,
    "num_removes": 0,
    "num_matches": 0,
    "cache_hit_rate": 0,
    "max_fanout": 0,
    "avg_fanout": 0,
    "stats_time": "2014-10-03T01:40:02.871104973+02:00"
  },
  "connections": [
    {
      "cid": 1,
      "ip": "127.0.0.1",
      "port": 51145,
      "pending_size": 0,
      "in_msgs": 0,
      "out_msgs": 0,
      "in_bytes": 0,
      "out_bytes": 0,
      "num_subscriptions": 1,
      "subscriptions": [
        "foo"
      ]
    }
  ]
}
```