Initial implementation of holiday/holiday calendar

An initial implementation of a holiday and calendars that are
collections these holidays.  Also add an Easter offset that will make
it easier to construct the Easter holiday.
Revert "New cython implementation of count for groupby."

This reverts commit 9e38e7fcc27e005b33b5cd76ab4cf72ac1c1d239.
Merge remote-tracking branch 'origin/holiday-calendar' into holiday-calendar