Added socket information.

Closes #180 

Adds `sockets` key under `load` key that looks something like this
```json
  "sockets": {
    "http": {
      "total":5,
      "www.google.com:80":5
    },
    "https":{
      "total":5,
      "www.google.com:443":5}
    }
}
```

In order to have a remote chance of properly tracing this, users need to supply a list of agents they want statistics on. For most people, it will be the global http and https agents. But the new `httpAgents` and `httpsAgents` are there in case you use something else.