Implement write tasks to log file

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

Why is this change necessary?
Tasks need to be logged for retrieval in future sessions. User can also
edit the log file directly instead of adding tasks through the program.

What side effects does this change have?
Changes to log file will be read, e.g. adding a task via the log file
will result in the task being read by the program. Such a change may
need to be synchronised to Google Calendar, and a need to detect such
changes may be needed.