Refactor node ID handling, use check digits (fixes #269)

New node ID:s contain four Luhn check digits and are grouped
differently. Code uses NodeID type instead of string, so it's formatted
homogenously everywhere.

luhn/luhn_test.go

@@ -0,0 +1,25 @@
+package luhn_test
+
+import (
+	"testing"
+
+	"github.com/calmh/syncthing/luhn"
+)
+
+func TestGenerate(t *testing.T) {
+	a := luhn.Alphabet("abcdef")
+	c := a.Generate("abcdef")
+	if c != 'e' {
+		t.Errorf("Incorrect check digit %c != e", c)
+	}
+}
+
+func TestValidate(t *testing.T) {
+	a := luhn.Alphabet("abcdef")
+	if !a.Validate("abcdefe") {
+		t.Errorf("Incorrect validation response for abcdefe")
+	}
+	if a.Validate("abcdefd") {
+		t.Errorf("Incorrect validation response for abcdefd")
+	}
+}