Add middleware/dnssec (#133)

This adds an online dnssec middleware. The middleware will sign
responses on the fly. Negative responses are signed with NSEC black
lies.

middleware/test/file.go

@@ -0,0 +1,20 @@
+package test
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+)
+
+// TempFile will create a temporary file on disk and returns the name and a cleanup function to remove it later.
+func TempFile(t *testing.T, dir, content string) (string, func(), error) {
+	f, err := ioutil.TempFile(dir, "go-test-tmpfile")
+	if err != nil {
+		return "", nil, err
+	}
+	if err := ioutil.WriteFile(f.Name(), []byte(content), 0644); err != nil {
+		return "", nil, err
+	}
+	rmFunc := func() { os.Remove(f.Name()) }
+	return f.Name(), rmFunc, nil
+}