start_date defines when a meeting series begins. start_date
is used in lieu of the current date when rebuilding the calendar.
This will ultimately let us move away from needing to have
biweekly-odd and biweekly-even.
I also added an example.yaml with a start_date
Change-Id: I0c5f6e5951b6c15b949f599182f48aa132b76abb
a8c789d4b4