Implement description field in Task class

Why is this change necessary?
- To store task descriptions keyed in by user.
- To allow for manipulation of task descriptions through methods of the
  Task class

How does it address the issue?
- So far, the task description can be initialised from Task's
  constructor.
- Setters and getters set (useful for edit functionality) and get task
  descriptions

What side effects does this change have?
- Task descriptions can now be added to tasks