Implement time extraction from task string

https://github.com/cs2103aug2014-f11-3j/main/issues/71

Why is this change necessary?
To read tasks (from a log file) into our software, one needs to read all
information about the task, including time.

How does it address the issue?
Extracting the time from a task string aids extraction from the text log
file itself; the log file can be read in as a string.