added TEST(OperatorPlus, MustBeCommutative)
added TEST(OperatorPlus, MustBeAssociative)
moved typedef for fraction_type to separate header-file
moved out to separate files FractionAlgorithms.h/.cpp all functions from Fraction.h/.cpp
moved out all tests for functions that are declared at FractionAlgorithms.h to separate file that is called FractionAlgorithmsTest.cpp