Added some extensions to .gitignore
Implement Date class

Issue #61 - https://github.com/cs2103aug2014-f11-3j/main/issues/61

Why is this change necessary?
Users may choose to enter dates for their tasks. So far, our Task class
handles only the task description without dates.

How does it address the issue?
Having a Date class stores the relevant fields, e.g. date, month, year,
and manipulates them using its methods.

What side effects does this change have?
Everything.
- Task object will need to have a Date object.
- GUI will have to parse the date and store it into a Date object.
- Logic will need this class while manipulating dates.
- Database needs to store the dates keyed in by user.
- Most importantly, GoogleCalendarManager now has a date to add events. You
  can't really add a floating task to Google Calender.