Implement Time class for Task object.

Issue #62 - https://github.com/cs2103aug2014-f11-3j/main/issues/62

Why is this change necessary?
User may key in time for their tasks, e.g. if Jim has an assignment
due on 1 Jan 2015, at precisely 2359 hrs, he needs to key in the time
11.59 pm as the "complete assignment" task's deadline (of course in
addition to the date of deadline). We need a class to store the
relevant time fields, e.g. hours and minutes, as well as relevant
getters and setters.

How does it address the issue?
Implementing the Time class stores and organises the time from user
input neatly into hours and minutes, and allows for manipulation of time
through its methods. A Time object should be included as a field of the
Task object, since a Task may have a time as its deadline.

What side effects does this change have?
Everything:
- UserInterface pass the time field to Logic
- Logic needs to manipulate time and store it in database.
- Database needs to store the time field of a task and synchronise with
  GoogleCalendarManager.
- GoogleCalendarManager needs to insert tasks into Google Calendar using
  the time field of tasks.

Close #62
Merge branch 'struct/time'