Add support to saveInstanceState and restoreInstanceState

[JonatanSalas]

To prevent the CalendarView to loose state when app restarts, we should implement the following methods inherited from View/ViewGroup:

• onSaveInstanceState
• onRestoreInstanceState

This would give the ability to:

• Not loose current month selected when exiting from app and coming back
• Not loose selection when coming back from one month to another
• In the future, this help to restore added events for month, when finish to implement the request feature under issue Is it possible add event on specific date? #23

This issue it's created in order to keep track of the changes made.