[BUILD] Added license check

1) Enforce license header check on source files
2) Add missing license header to existing sources

Original commit: elastic/x-pack-elasticsearch@da73e4f2b6

dev-tools/elasticsearch_license_header.txt

@@ -0,0 +1,14 @@
+ELASTICSEARCH CONFIDENTIAL
+__________________
+
+ [2014] Elasticsearch Incorporated. All Rights Reserved.
+
+NOTICE:  All information contained herein is, and remains
+the property of Elasticsearch Incorporated and its suppliers,
+if any.  The intellectual and technical concepts contained
+herein are proprietary to Elasticsearch Incorporated
+and its suppliers and may be covered by U.S. and Foreign Patents,
+patents in process, and are protected by trade secret or copyright law.
+Dissemination of this information or reproduction of this material
+is strictly forbidden unless prior written permission is obtained
+from Elasticsearch Incorporated.

dev-tools/license_header_definition.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<additionalHeaders>
+    <javadoc_style>
+        <firstLine>/*</firstLine>
+        <beforeEachLine> * </beforeEachLine>
+        <endLine> */EOL</endLine>
+        <!--skipLine></skipLine-->
+        <firstLineDetectionPattern>(\s|\t)*/\*.*$</firstLineDetectionPattern>
+        <lastLineDetectionPattern>.*\*/(\s|\t)*$</lastLineDetectionPattern>
+        <allowBlankLines>false</allowBlankLines>
+        <isMultiline>true</isMultiline>
+    </javadoc_style>
+</additionalHeaders>

pom.xml

@@ -432,6 +432,33 @@
                             <argLine>${tests.jvm.argline}</argLine>
                         </configuration>
                     </plugin>
+                    <plugin>
+                      <groupId>com.mycila</groupId>
+                      <artifactId>license-maven-plugin</artifactId>
+                      <version>2.5</version>
+                      <configuration>
+                        <header>dev-tools/elasticsearch_license_header.txt</header>
+                        <headerDefinitions>
+                          <headerDefinition>dev-tools/license_header_definition.xml</headerDefinition>
+                        </headerDefinitions>
+                        <includes>
+                          <include>src/main/java/org/elasticsearch/**/*.java</include>
+                          <include>src/test/java/org/elasticsearch/**/*.java</include>
+                        </includes>
+                        <excludes>
+                          <!-- BCrypt -->
+                          <exclude>src/main/java/org/elasticsearch/shield/authc/support/BCrypt.java</exclude>
+                        </excludes>
+                      </configuration>
+                      <executions>
+                        <execution>
+                          <phase>compile</phase>
+                          <goals>
+                            <goal>check</goal>
+                          </goals>
+                        </execution>
+                      </executions>
+                    </plugin>
                 </plugins>
             </build>
         </profile>