* Fix of the issue that choosing French
or Spanish as a system language
resulted in no input language selected.
* Added keyboard input parser unit
tests for French and Spanish input
maps.
* Minor cleanups.
* Fixed issue with subtract operation triggering unusal behavior.
* Unified handling of the add,sub,mul,div operations.
* Updated and verified existing unit tests and added new ones covering
unhandled cases.
* Added basic requirements/readme.
Modify calculator behavior according to PO input:
* Compute existing equation before entering next operation
* Limit digit input to 7 digits
* Start each number input from empty input
