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/whoami/setup_test.go

@@ -0,0 +1,19 @@
+package whoami
+
+import (
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+func TestSetupWhoami(t *testing.T) {
+	c := caddy.NewTestController("dns", `whoami`)
+	if err := setupWhoami(c); err != nil {
+		t.Fatalf("Expected no errors, but got: %v", err)
+	}
+
+	c = caddy.NewTestController("dns", `whoami example.org`)
+	if err := setupWhoami(c); err == nil {
+		t.Fatalf("Expected errors, but got: %v", err)
+	}
+}