Version 0.6.16

+ Minimum required version of u2py updated to 1.6.55.1 due to
  changes in interface_basis.load functionality.
  Interface module now makes use of new parameter of interface_basis.load function
  to check function arguments and maintain correct Terminal state.
  Due to this change, it's now possible to tolerate temporary disappearance
  of unix stream socket and reconnect to it after its recovery.
+ interface module got notifications on card operations.
+ grabbing terminal titles from remote database is now performed inside mainloop
  thread to avoid possible user interface freeze.
+ terminals.ui got progress bar to display current number of free places for parking.
+ db.DB now inherits QObject and provides free_places_update signal.
+ Added basic implementation for payments module.