Activate checkstyle and add missing license headers

Delete also all "All rights reserved" statements, as they are
unnecessary and conflict with checkstyle's header check. Delete unused
imports.

experimental/src/main/java/org/jivesoftware/smackx/ExperimentalProviderInitializer.java

@@ -1,7 +1,8 @@
 /**
+ *
  * Copyright 2013 Robin Collier
  *
- * All rights reserved. Licensed under the Apache License, Version 2.0 (the "License");
+ * 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
  *
@@ -13,7 +14,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-
 package org.jivesoftware.smackx;
 
 import org.jivesoftware.smack.initializer.UrlProviderFileInitializer;

experimental/src/main/java/org/jivesoftware/smackx/carbons/CarbonManager.java

@@ -1,7 +1,8 @@
 /**
+ *
  * Copyright 2013 Georg Lukas
  *
- * All rights reserved. Licensed under the Apache License, Version 2.0 (the "License");
+ * 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
  *
@@ -13,7 +14,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-
 package org.jivesoftware.smackx.carbons;
 
 import java.lang.ref.WeakReference;

experimental/src/main/java/org/jivesoftware/smackx/carbons/packet/CarbonExtension.java

@@ -1,7 +1,8 @@
 /**
+ *
  * Copyright 2013 Georg Lukas
  *
- * All rights reserved. Licensed under the Apache License, Version 2.0 (the "License");
+ * 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
  *
@@ -13,7 +14,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-
 package org.jivesoftware.smackx.carbons.packet;
 
 import org.jivesoftware.smack.packet.PacketExtension;

experimental/src/main/java/org/jivesoftware/smackx/carbons/provider/CarbonManagerProvider.java

@@ -1,7 +1,8 @@
 /**
+ *
  * Copyright 2013 Georg Lukas
  *
- * All rights reserved. Licensed under the Apache License, Version 2.0 (the "License");
+ * 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
  *
@@ -13,7 +14,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-
 package org.jivesoftware.smackx.carbons.provider;
 
 import org.jivesoftware.smack.packet.PacketExtension;

experimental/src/test/java/org/jivesoftware/smackx/ExperimentalProviderInitializerTest.java

@@ -1,3 +1,19 @@
+/**
+ *
+ * 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;
 
 import static org.junit.Assert.assertTrue;

experimental/src/test/java/org/jivesoftware/smackx/carbons/CarbonTest.java

@@ -1,5 +1,8 @@
 /**
- * All rights reserved. Licensed under the Apache License, Version 2.0 (the "License");
+ *
+ * Copyright the original author or authors
+ *
+ * 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
  *