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.

Paging Dr. @lloydbenson in case you want to look this before it gets merged in.
I had to drop `annotate()` here because it chokes on the `http.Agent` object due to circular references.