Create Recurring rent model

* 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.
* I also need to find a way to have rent paid for a certain
  billing period and stop it from being double paid... also
  set the rent status as paid when the renter pays the rent