Add checkstyle support

Fixes #481

Execute checkstyle validation at build time.
Add minimal set of rules.
Ensure that existing code conforms to the rules.

pom.xml

@@ -19,6 +19,7 @@
 	</scm>
 	<properties>
 		<java.version>1.7</java.version>
+		<disable.checks>false</disable.checks>
 	</properties>
 	<dependencyManagement>
 		<dependencies>
@@ -46,6 +47,21 @@
 	<build>
 		<pluginManagement>
 			<plugins>
+				<plugin>
+					<groupId>org.apache.maven.plugins</groupId>
+					<artifactId>maven-checkstyle-plugin</artifactId>
+					<version>2.17</version>
+					<configuration>
+						<configLocation>src/checkstyle/checkstyle.xml</configLocation>
+					</configuration>
+					<dependencies>
+						<dependency>
+							<groupId>com.puppycrawl.tools</groupId>
+							<artifactId>checkstyle</artifactId>
+							<version>6.17</version>
+						</dependency>
+					</dependencies>
+				</plugin>
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>
 					<artifactId>maven-javadoc-plugin</artifactId>
@@ -62,6 +78,31 @@
 				</plugin>
 			</plugins>
 		</pluginManagement>
+		<plugins>
+			<plugin>
+				<groupId>org.apache.maven.plugins</groupId>
+				<artifactId>maven-checkstyle-plugin</artifactId>
+				<executions>
+					<execution>
+						<id>checkstyle-validation</id>
+						<phase>validate</phase>
+						<configuration>
+							<skip>${disable.checks}</skip>
+							<configLocation>src/checkstyle/checkstyle.xml</configLocation>
+							<headerLocation>src/checkstyle/checkstyle-header.txt</headerLocation>
+							<propertyExpansion>checkstyle.build.directory=${project.build.directory}</propertyExpansion>
+							<encoding>UTF-8</encoding>
+							<consoleOutput>true</consoleOutput>
+							<failsOnError>true</failsOnError>
+							<includeTestSourceDirectory>true</includeTestSourceDirectory>
+						</configuration>
+						<goals>
+							<goal>check</goal>
+						</goals>
+					</execution>
+				</executions>
+			</plugin>
+		</plugins>
 	</build>
 	<profiles>
 		<profile>