Add ListenAndServe for transparent proxy support

Instead of:

```go
	if os.Getenv("LAMBDA_FUNCTION_NAME") == "" {
		log.Fatal(http.ListenAndServe(":5555", h))
	} else {
		apex.Handle(proxy.Serve(h))
	}
```

Just do:

```go
log.Fatal(apex.ListenAndServe(":5555", h))
```

and let this pkg handle that