Rework XMPP Date/Time related code

- Fix "packet.Time is not thread-safe" (SMACK-543)
- Update packet.Time to XEP-0202

Add SDM.supportsFeature(), since this is a pattern that repeats over and
over again in Smack. Also add abstract Manager class, that takes care of
the weak reference to Connection, as this is also a repeating pattern in
Smack.

extensions/src/test/java/org/jivesoftware/smackx/InitExtensions.java

@@ -0,0 +1,26 @@
+/**
+ *
+ * Copyright 2014 Florian Schmaus
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.jivesoftware.smackx;
+
+public class InitExtensions {
+
+    static {
+        (new ExtensionsProviderInitializer()).initialize();
+        (new ExtensionsStartupClasses()).initialize();
+    }
+
+}