Some tweaks and fixes for Common::Rational

* Fix Common::gcd to work with negative input
* This fixes a bug in Common::Rational's multiplication code
* Add some more basic unit tests (including one which checks for
  the now fixed multiplication bug)
* cleanup

svn-id: r49064

common/algorithm.h

@@ -227,6 +227,8 @@ void sort(T first, T last) {
  */
 template<class T>
 T gcd(T a, T b) {
+	if (a <= 0) a = -a;
+	if (b <= 0) b = -b;
 	while (a > 0) {
 		T tmp = a;
 		a = b % a;