Added Google Firebase

app/src/main/java/de/sebse/fuplanner/services/Canteen/types/Canteen.java

@@ -79,7 +79,7 @@ public class Canteen implements Serializable, Iterable<Day> {
         int m = calendar.get(Calendar.MONTH)+1;
         int d = calendar.get(Calendar.DAY_OF_MONTH);
 
-        return String.format(Locale.getDefault(), "%d-%d-%d", y, m, d);
+        return String.format(Locale.getDefault(), "%4d-%2d-%2d", y, m, d).replace(' ', '0');
     }
 
     public static Calendar keyToCalendar(String dateString) {

app/src/main/java/de/sebse/fuplanner/tools/SortedListDay.java

@@ -8,7 +8,6 @@ import de.sebse.fuplanner.services.Canteen.types.Day;
 public class SortedListDay extends SortedList<Day, Calendar, String> {
     @Override
     int compare(Day o1, Day o2) {
-        // TODO correct implementation
         return Canteen.calendarToKey(o1.getCalendar()).compareTo(Canteen.calendarToKey(o2.getCalendar()));
     }