Improve crypto

- Re-organize packages to be consistent
- Add Key store tests

src/main/java/io/kamax/mxisd/crypto/Signature.java

@@ -0,0 +1,29 @@
+/*
+ * mxisd - Matrix Identity Server Daemon
+ * Copyright (C) 2019 Kamax Sàrl
+ *
+ * https://www.kamax.io/
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package io.kamax.mxisd.crypto;
+
+public interface Signature {
+
+    KeyIdentifier getKey();
+
+    String getSignature();
+
+}