Support http.CloseNotifier interface.

Add support for the [CloseNotifier](http://golang.org/pkg/net/http/#CloseNotifier) interface.

Without this code, attempts to cast to `CloseNotifier` die with:

```
panic: interface conversion: *martini.responseWriter is not http.CloseNotifier: missing method CloseNotify [recovered]
	panic: interface conversion: *martini.responseWriter is not http.CloseNotifier: missing method CloseNotify
```