Add checkstyle plugin

Also made root pom parent pom of submodules which
was supposed to go into the last commit

Author: Bill Prin

cloud-storage/xml-api/cmdline-sample/pom.xml

@@ -1,5 +1,14 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <project>
+
+  <parent>
+    <artifactId>doc-samples</artifactId>
+    <groupId>com.google.cloud</groupId>
+    <version>1.0.0</version>
+    <relativePath>../..</relativePath>
+  </parent>
+
+
   <modelVersion>4.0.0</modelVersion>
   <groupId>com.google.apis-samples</groupId>
   <artifactId>storage-cmd-line-sample</artifactId>

cloud-storage/xml-api/serviceaccount-appengine-sample/pom.xml

@@ -1,11 +1,14 @@
 <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
   <modelVersion>4.0.0</modelVersion>
+
   <parent>
-    <groupId>com.google</groupId>
-    <artifactId>google</artifactId>
-    <version>5</version>
+    <artifactId>doc-samples</artifactId>
+    <groupId>com.google.cloud</groupId>
+    <version>1.0.0</version>
+    <relativePath>../..</relativePath>
   </parent>
+  
   <groupId>com.google.apis-samples</groupId>
   <artifactId>serviceaccounts-appengine-sample</artifactId>
   <version>1.0.0</version>
@@ -108,14 +111,14 @@
     </plugins>
   </build>
   <dependencies>
-    
+
     <!-- Google App Engine SDK -->
     <dependency>
       <groupId>com.google.appengine</groupId>
       <artifactId>appengine-api-1.0-sdk</artifactId>
       <version>${project.appengine.version}</version>
     </dependency>
-    
+
     <dependency>
       <groupId>com.google.api-client</groupId>
       <artifactId>google-api-client-appengine</artifactId>

pom.xml

@@ -22,8 +22,32 @@
         <module>taskqueue/deferred</module>
         <module>unittests</module>
         <module>bigquery</module>
+        <module>cloud-storage/xml-api/cmdline-sample</module>
+        <module>cloud-storage/xml-api/serviceaccount-appengine-sample</module>
     </modules>
 
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-checkstyle-plugin</artifactId>
+                <version>2.9.1</version>
+                <executions>
+                    <execution>
+                        <id>checkstyle</id>
+                        <phase>validate</phase>
+                        <goals>
+                            <goal>check</goal>
+                        </goals>
+                        <configuration>
+                            <failOnViolation>true</failOnViolation>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+
     <dependencyManagement>
         <dependencies>
         <!-- Compile/runtime dependencies -->