Updating the persistence info for problems
Showing creation of new list, closing and seeing if list is there when reopening
Merge pull request #8 from tblazhev/master

Updating the persistence info for problems
Adding remove_subscriber command to mail problem
Adding update_subscriber command to the problem
Adding update command for lists to the problem
Adding delete command for lists to the problem
Adding import json command to the problem
Adding materials for week4 - SQL
Adding links for sqlite
Adding problem set 1 for week 4

- SQL and sqlite related tasks
Typo fix: aswered -> answered
Adding ; at the end of INSERT
Merge remote-tracking branches 'origin' and 'upstream/master'
Fixing the monthly and yearly sum
Merge remote-tracking branches 'origin' and 'upstream/master'