Calculator

Task:

• Create a simple calculator, that can "+", "-", "*", "/", "clear", and "="
• Model, logic and UI must be separated
• Unit tests must be implemnted

Possible improvements:

• add possibility to enter float values
• hihlight selected mathematical operator
• add possibility to reverse sign of the value
• add iPad support
• add possibility to execute multiple arithmetic command in one line
• add UI test
• add more mathematical operators