Configure the build to use Checkstyle

This commit adds checkstyle plugin and provides a simple config file.

Closes gh-125

docs/src/test/java/com/example/ExampleApplicationTests.java

@@ -27,7 +27,7 @@ import org.springframework.web.context.WebApplicationContext;
 import static org.springframework.restdocs.mockmvc.MockMvcRestDocumentation.documentationConfiguration;
 
 public class ExampleApplicationTests {
-	
+
 	// tag::mock-mvc-setup[]
 	@Rule
 	public final RestDocumentation restDocumentation = new RestDocumentation("build/generated-snippets");
@@ -44,4 +44,4 @@ public class ExampleApplicationTests {
 				.build();
 	}
 	// end::mock-mvc-setup[]
-}
+}