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.

config/checkstyle.xml

@@ -1,9 +1,29 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE module PUBLIC
-    "-//Puppy Crawl//DTD Check Configuration 1.3//EN" 
-     "http://www.puppycrawl.com/dtds/configuration_1_3.dtd"> 
+    "-//Puppy Crawl//DTD Check Configuration 1.3//EN"
+     "http://www.puppycrawl.com/dtds/configuration_1_3.dtd">
 <module name="Checker">
+	<module name="SuppressionFilter">
+		<property name="file" value="config/suppressions.xml"/>
+	</module>
+	<module name="Header">
+		<property name="headerFile" value="config/header.txt"/>
+		<property name="ignoreLines" value="3"/>
+		<property name="fileExtensions" value="java"/>
+	</module>
 	<module name="TreeWalker">
-		<module name="UnusedImports"/> 
+		<module name="UnusedImports">
+			<property name="processJavadoc" value="true"/>
+		</module>
+		<!-- TODO: enable later on
+		<module name="AvoidStarImport"/>
+		<module name="IllegalImport"/>
+		<module name="RedundantImport"/>
+		<module name="UpperEll"/>
+		<module name="ArrayTypeStyle"/>
+		<module name="GenericWhitespace"/>
+		<module name="EmptyStatement"/>
+		<module name="PackageDeclaration"/>
+		-->
 	</module>
 </module>

config/header.txt

@@ -0,0 +1,16 @@
+/**
+ *
+ * Copyright 20XX 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
+ *
+ *     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.
+ */

config/suppressions.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0"?>
+<!DOCTYPE suppressions PUBLIC
+    "-//Puppy Crawl//DTD Suppressions 1.1//EN"
+    "http://www.puppycrawl.com/dtds/suppressions_1_1.dtd">
+<suppressions>
+	<!-- currently none -->
+</suppressions>