Version 0.11.6

+ Added db.DB test module with db.db.generate_payment
+ config module got new constant: DATETIME_FORMAT_FULL.
  It is designed to replace DATETIME_FORMAT in cases where there is no auto prepending
  `20` to column values.
+ refactored database payment generation: every payment should now provide db_payments_args
  property that returns a dictionary with predefined keys.
+ db.DB.query now generates notification and returns False instead of Exception
  when there was FAIL response from remote database.
+ introduced BaseTicketPayment that serves as a common code base and base class
  for TicketPayment and TicketExcessPayment.
+ fixed ticket payments execution errors introduced in previous version.
+ ticket tariff results should now properly handle zero-unit payment and adjust paid_time
  appropriately to the delta.