Use reflection instead of string class name

[VaclavSir]

I don't really like this line from the readme:

$addition = new AdditionOperator('SimplePHPEasyPlus\Number\SimpleNumber');

String constants are almost as bad code smell as the addition operator. SimpleNumber class should provide a static method that returns its reflection. Something like this:

public static function getReflection()
{
    return new ReflectionClass(get_called_class());
}

AdditionOperator could use ReflectionClass::newInstance() instead of new $resultClass() and the line mentioned above would look much cleaner then:

$addition = new AdditionOperator(SimpleNumber::getReflection());