Add Checkstyle to build

Fixes gh-393

gradle/java.gradle

@@ -6,6 +6,7 @@ apply plugin: 'propdeps'
 apply plugin: 'propdeps-idea'
 apply plugin: 'propdeps-eclipse'
 apply plugin: 'com.github.ben-manes.versions'
+apply plugin: 'checkstyle'
 
 group = 'org.springframework.session'
 
@@ -75,6 +76,12 @@ task integrationTest(type: Test, dependsOn: jar) {
 }
 check.dependsOn integrationTest
 
+checkstyle {
+	configFile = rootProject.file('config/checkstyle/checkstyle.xml')
+	configProperties.configDir = configFile.parentFile
+	toolVersion = '6.16.1'
+}
+
 eclipse {
 	classpath {
 		plusConfigurations += [ configurations.integrationTestCompile ]
@@ -83,4 +90,4 @@ eclipse {
 
 project.idea.module {
 	scopes.TEST.plus += [project.configurations.integrationTestRuntime]
-}
+}