Clear appointment cache daily

[dolsysmith]

In order to support recurring appointments, we need to clear the appts table on a regular basis. (Recurring appointments from LibCal use the same ID, so the app currently discards any instances of a recurring appointment after the first.)

Proposed implementation:

• add a method to sqlite_cache.py to clear the appts table.
• in app.py add a new function to call the delete method and schedule itself for 24 hours from the current time
• on startup, the script will need to calculate the time to midnight and schedule the above for that initial delay

[dolsysmith]

Need to fix error that occurs at the end of the month:

ValueError: day is out of range for month
Traceback (most recent call last):
  File "/opt/libcal_pp_integration/app.py", line 221, in <module>
    scheduler.enterabs(get_next_midnight(), 2, cleanup, argument=(app, scheduler))
  File "/opt/libcal_pp_integration/app.py", line 200, in get_next_midnight
    midnight = datetime(today.year, today.month, today.day+1, 0, 0, 0).timestamp()