Implement toString method for Calendar objects

Why is this change necessary?
- To log tasks in a text file, all information about the tasks must be
  written explicitly in the log file.

How does it address the issue?
- To write the start and end dates and times of a task, I need to
  convert the Calendar object to a String object.

What side effects does this change have?
- GUI may want to use this toString method for display of task
  deadlines.