May choose if CalendarMonths show past or future months (if more than one)

[andywer]

Use case: If you show multiple calendar months (let's say we have got two) than there are basically two options:

• You focus on the past, so the first CalendarMonth should show the last month and second should show the current one
• You focus on the future, so the first CalendarMonth should show the current month and second should show the next one

This PR introduces the showCurrentMonth prop to achieve that:
showCurrentMonth: React.PropTypes.oneOf(['first', 'last'])