Add apache v2 license header

pom.xml

@@ -14,6 +14,7 @@
         described by Hugo Krawczyk. It is special as it supports 2 Phases: Extract and Expand.
     </description>
     <url>https://github.com/patrickfav/hkdf</url>
+    <inceptionYear>2017</inceptionYear>
 
     <prerequisites>
         <maven>3.1.0</maven>

src/main/java/at/favre/lib/crypto/HKDF.java

@@ -1,3 +1,18 @@
+/*
+ * Copyright 2017 Patrick Favre-Bulle
+ *
+ * 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 at.favre.lib.crypto;
 
 import javax.crypto.Mac;

src/main/java/at/favre/lib/crypto/HkdfMacFactory.java

@@ -1,3 +1,18 @@
+/*
+ * Copyright 2017 Patrick Favre-Bulle
+ *
+ * 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 at.favre.lib.crypto;
 
 import javax.crypto.Mac;