Refactor substrings in displayTask method

https://github.com/cs2103aug2014-f11-3j/main/issues/70

Why is this change necessary?
Refactoring (specifically extracting the substrings as separate
functions) allows for:
- Neater code.
- Better re-usability, as in other developers can use displayTitle
  method if they just want to display the task title on-screen, as
  opposed to display all the attributes of the task.

What side effects does this change have?
Other developers can now selectively convert certain information about
the task, e.g. just the title, to strings. May be useful for GUI.