created System Class diagram

Signed-off-by: Stanislav Seletskiy <leostas9@gmail.com>
Corrected system class diagram

Signed-off-by: Stanislav Seletskiy <leostas9@gmail.com>
Created operations for IBookingSystem & BookingSystem

Signed-off-by: Stanislav Seletskiy <leostas9@gmail.com>