Added getRevision() helper function, refactored revision constant

CloudObjects/SDK/Constants.php

@@ -0,0 +1,13 @@
+<?php
+
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+ 
+namespace CloudObjects\SDK;
+
+class Constants {
+
+    const PROPERTY_REVISION = 'coid://cloudobjects.io/isAtRevision';
+
+}