Pad non-padded secrets. This lets us continue building on <= go1.8.

- Add tests for secrets using various padding methods.
- Add a new method/test to append padding to non-padded secrets.

util.go

@@ -0,0 +1,16 @@
+package twofactor
+
+import (
+	"strings"
+)
+
+// Pad calculates the number of '='s to add to our encoded string
+// to make base32.StdEncoding.DecodeString happy
+func Pad(s string) string {
+	if !strings.HasSuffix(s, "=") && len(s)%8 != 0 {
+		for len(s)%8 != 0 {
+			s += "="
+		}
+	}
+	return s
+}