Added Logger interface and FakeLogger
Added constructor with ILogger parameter
Replaced data fields with data accessors
Implemented ViewModelLoggingLogic
Refactoring for null checks inViewModel log.
Implemented TxtFileLogger with integration tests
Fixed test log file name for correct .gitignore work
Added new constructor to viewModel
Added view model logging to UI
NullSafeWaraper refactored to Null logger.
Merge pull request #102 from Ruszrok/lab3-currency-converter-logger

[KK] Yudanov -- Lab3 currency converter logger. Added Null Logger instead of NullLoggerWrapper. Ready to merge.