Use correct Base64 dialect when manually parsing JWTs

JWTs use URL-encoded base64 (with - _ instead of + /). This fixes parsing bugs
that would occur when issuing attributes containing certain non-alphanumeric
characters like ~.

Fixes #24.
Co-authored-by: Koen van Ingen <koenvaningen@gmail.com>

messages.go

@@ -275,7 +275,7 @@ func JwtDecode(jwt string, body interface{}) error {
 	if jwtparts == nil || len(jwtparts) < 2 {
 		return errors.New("Not a JWT")
 	}
-	bodybytes, err := base64.RawStdEncoding.DecodeString(jwtparts[1])
+	bodybytes, err := base64.RawURLEncoding.DecodeString(jwtparts[1])
 	if err != nil {
 		return err
 	}