Added implementation for supporting following Calendar API and their specs:

* Calendar.for_week(4).of_month(8).in_year(2012)
* Calendar.for_month(6).in_year(2011)
* Calendar.for_year(2014)
* Calendar.of_month(5)
* Calendar.for_current_week
* Calendar.for_current_month
* Calendar.for_current_year

Handled following invalid usages:

* Calendar.for_week(4).show
* Calendar.for_month(7).show

Note: The show implementation is not complete yet and more specs are required to be added esp the main implementation for generating the calendar is pending.