Change timestamp to int

Simpler for arithmetic and whatnot. We will convert it to mm/dd/yyyy
for the view as needed.
Faster faster faster

Added pks, fks, and indices to the tables. Also added the ability to
read punches by date to only run calculations on the relevant punches.
Attendance calculations by month

Model now calculates and stores attendance data by month for each
child. Attendance unit is minutes or days, depending on the child's
config.