Implement all extractions from split strings

https://github.com/cs2103aug2014-f11-3j/main/issues/57

Why is this change necessary?
Tasks need to be read in from task log.

How does it address the issue?
The task log is typically read in line by line by a buffered reader to a
string representing all information about a single task. If we call this
string a task string and split it into substrings holding information
about various fields of a task, then it is only natural that we parse
these substrings into their appropriate types, e.g. Calendar object from
a startTime substring, for the direct setting via the task's setters.
This is a step towards creating a Task object from reading strings from
the log file.

What side effects does this change have?
Setters can now be directly deployed to set information about a newly
created task, essentially achieving the ability to read and create tasks
from the task log.