Adjust checkstyle rules. Make checkstyle fail the build when violations are found. Correct all current checkstyle violations.

memento/src/main/java/com/iluwatar/memento/App.java

@@ -23,6 +23,9 @@ import java.util.Stack;
  */
 public class App {
 
+  /**
+   * Program entry point
+   */
   public static void main(String[] args) {
     Stack<StarMemento> states = new Stack<>();
 

memento/src/main/java/com/iluwatar/memento/Star.java

@@ -11,12 +11,18 @@ public class Star {
   private int ageYears;
   private int massTons;
 
+  /**
+   * Constructor
+   */
   public Star(StarType startType, int startAge, int startMass) {
     this.type = startType;
     this.ageYears = startAge;
     this.massTons = startMass;
   }
 
+  /**
+   * Makes time pass for the star
+   */
   public void timePasses() {
     ageYears *= 2;
     massTons *= 8;