Add missing @Override annotations

This commit also adds MissingOverrideCheck module to Checkstyle configuration.

samples/xml/dms/src/main/java/sample/dms/Directory.java

@@ -31,6 +31,7 @@ public class Directory extends AbstractElement {
 		super(name, parent);
 	}
 
+	@Override
 	public String toString() {
 		return "Directory[fullName='" + getFullName() + "'; name='" + getName()
 				+ "'; id='" + getId() + "'; parent='" + getParent() + "']";

samples/xml/dms/src/main/java/sample/dms/File.java

@@ -39,6 +39,7 @@ public class File extends AbstractElement {
 		this.content = content;
 	}
 
+	@Override
 	public String toString() {
 		return "File[fullName='" + getFullName() + "'; name='" + getName() + "'; id='"
 				+ getId() + "'; content=" + getContent() + "'; parent='" + getParent()