Recurring due date

* Create first steps for recurring due date 
* Set up initial recurring rent payments
* Set up cron job to check whose rent is due
  and if so create a rent object for that month.
* I need to test this further and then begin writing
  tests for this.
* Add due date and update cron jobs 
* Fix rake tasks to only create a new rent instance for the
  current rent instance if the current rent for that billing period
  hasn't been created yet