[Cs3] airline booking lab

[Mikhail Nesterenko]

CS3 students,

I updated the airline booking sample code to correctly report
duplicates. I used a vector of pairs to ensure uniqueness of 
of flight/seat combination and then iterate over these pairs and
for each pair, I iterate over all the ticket records to make sure
there are no duplicates. 

I also updated the log.txt file to check for all possible duplication
cases.

Thanks,
-- 
Mikhail