Remove Out variable in Response struct and embed http.ResponseWriter

This changes the Response struct to embed the http.ResponseWriter. This causes a name conflict with ResponseWriter.WriterHeader and the following error:
```
*Response does not implement http.ResponseWriter (wrong type for WriteHeader method)
        have WriteHeader(int, string)
        want WriteHeader(int)
```
I've renamed the WriteHeader to WriteStatusHeader to avoid the conflict. The other changes are to replace the Response.Out code.