added definition for operator+()
added isEmpty() method
added isNull() method
Added notes about empty and null matrixes.
changed name of test MatrixMultiplicateWithAnotherMatrix to
MatrixMultiplicateWithVector
added case Matrices2x2 to MatrixAddWithAnotherMatrix test
added case MatricesWithDifferentSizes to test MatrixAddWithAnotherMatrix
added case AllCases to test MatrixIsEmpty
added method isSquare()
added method isZeroMatrix()
added method haveSameSize() to private section
added case AllCases to MatrixIsNull test
is operator+() definition replaced

if(getRowsNumber()      != operand.getRowsNumber()      ||
-           getColumnsNumber()   != operand.getColumnsNumber()   )

to

!haveSameSize(operand)
added case NullMatrix to test MatrixIsZero
added case EmptyMatrix to test MatrixIsZero
added case ZeroMatrix to test MatrixIsZero
added case NonZeroMatrix to test MatrixIsZero
added definition for unary operator-
added test for unary operator-