middleware/whoami: add (#264)

Add a new middleware that tells you who you are; IP, port and transport
is echoed back.

Also some various cleanup and documentation improvements while at it:

* ResponseWriter: improve the documentation of these helper functions.
* And add an NextHandler for use in tests. Make chaos_test.go and
* whoam_test.go use it.

middleware/test/responsewriter.go

@@ -6,14 +6,19 @@ import (
 	"github.com/miekg/dns"
 )
 
+// ResponseWriter is useful for writing tests. It uses some fixed values for the client. The
+// remote will always be 10.240.0.1 and port 40212. The local address is always 127.0.0.1 and
+// port 53.
 type ResponseWriter struct{}
 
+// LocalAddr returns the local address, always 127.0.0.1:53 (UDP).
 func (t *ResponseWriter) LocalAddr() net.Addr {
 	ip := net.ParseIP("127.0.0.1")
 	port := 53
 	return &net.UDPAddr{IP: ip, Port: port, Zone: ""}
 }
 
+// RemoteAddr returns the remote address, always 10.240.0.1:40212 (UDP).
 func (t *ResponseWriter) RemoteAddr() net.Addr {
 	ip := net.ParseIP("10.240.0.1")
 	port := 40212