Tests now use JUnit 4 again
Refactored Calculation to use CalculationEntry

Calculation used an interface (CalculationItem) and two implementing
classes (CalculationOperation and CalculationNumber) to store
calculation entries. This led to many casts and plenty of messy code.

This refactoring removes the need for casts just to retrieve data from
calculation entries.
Calculation refactoring
Calculation test refactoring
Refactored Calculation "apply op" code
Fraction test addition