Implement add task to temporary memory

Issue #69 - https://github.com/cs2103aug2014-f11-3j/main/issues/69

Why is this change necessary?
Addition of a task consists 2 main parts:
- Add to a temporary memory stored by the program, e.g. add to
  arraylist that will be erased after the program exits.
- Store the temporary memory into a text file.

How does it address the issue?
The first point in the above paragraph is implemented.

Close issue #69