Add sort command

Allow user to sort the list base on the task's completion status.

Let's
* add SortCommand.java
* update Parser.java to include "sort"

src/main/java/command/SortCommand.java

@@ -0,0 +1,58 @@
+package command;
+
+import java.util.Comparator;
+
+import task.Task;
+import task.TaskList;
+
+/**
+ * {@inheritDocs}
+ * Sort the tasks in the task list.
+ */
+public class SortCommand extends Command {
+    private TaskList taskList;
+
+    /**
+     * Creates an instance of SortCommand.
+     */
+    public SortCommand(TaskList taskList) {
+        this.taskList = taskList;
+    }
+
+    /**
+     * {@inheritDocs}
+     * List all the tasks in the task list.
+     */
+    @Override
+    public String execute() {
+        taskList.getList().sort(new StatusComparator());
+        String message = "I have sorted the task list according to the tasks' statuses.";
+        return message + "\n\n" + new ListCommand(taskList).execute();
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * @return True if program will exit.
+     */
+    @Override
+    public boolean isExit() {
+        return false;
+    }
+
+    static class StatusComparator implements Comparator<Task> {
+        // undone status are on top of the list
+        @Override
+        public int compare(Task task1, Task task2) {
+            if (!task1.checkStatus() && task2.checkStatus()) {
+                return -1;
+
+            } else if (task1.checkStatus() && !task2.checkStatus()) {
+                return 1;
+
+            } else {
+                return task1.getTaskName().compareToIgnoreCase(task2.getTaskName());
+            }
+        }
+    }
+}

src/main/java/common/Parser.java

@@ -10,6 +10,7 @@
 import command.FindCommand;
 import command.ListCommand;
 import command.MarkCommand;
+import command.SortCommand;
 import command.UnmarkCommand;
 import task.TaskList;
 
@@ -76,12 +77,16 @@ public Command parse() throws IndexOutOfBoundsException, NumberFormatException,
             cmd = new FindCommand(tasks, st);
             return cmd;
 
+        case "sort":
+            cmd = new SortCommand(tasks);
+            return cmd;
+
         case "bye":
             cmd = new ExitCommand();
             return cmd;
 
         default:
-            throw new DukeException("OOPS!! Pls try again. :)");
+            throw new DukeException("OOPS!! Pls try again, hustler. :)");
         }
     }
 }