Add recording JSON feature

I will support new feature for stubcell.

stubcell will have a recording json feature!!!
this feature has 2 functions.

1. request proxy to backend server
2. record backend server response

so if we does not have an JSON, stubcell delegates the request to backend server, and serves JSON from backend server. And store the JSON to local file.

If you would like to setup the recording feature. you need to set the following settings.

``` js
stubcell.loadEntry(__dirname + "/example.yaml", {
  record: {
    // proxy server path
    proxy : "http://twitter.com",
  }
});
```

that's only cool!!