Version 0.11.1

Multiple improvements and bugfixes (not tested yet).

+ db.DB.generate_payment should now properly set operator name for payments.
+ notifications about paper status have been temporarily disabled.
+ payments.Payments.end_session now correctly disconnects from new_payable signal.
+ multiple changes in tariff module:
  + improved docstrings for Tariff class.
  + implemented new attribute for tariff results related to tickets: paid_time
    this attribute determines client paid time.
    This attribute introduces a new feature that conforms to common logic:
    client can use whole time that he paid for, not limited to FREE_TIME interval
    just after payment has been executed.
  + updated all doctests related to aforementioned `paid_time` attribute introducing.
  + DynamicTariff now explicitly raises ValueError when its interval is not Tariff.HOURLY.
  + minor update of test_tariff module.
+ ticket module payments now use `time_paid` attribute from ticket-related tariff results.
  Paid time information should now be correctly reflected in database and client checks.