- added ex::to_rational() to convert general expression to rational expression

- added ex::to_rational() to convert general expression to rational expression
  by replacing all non-rational parts with temporary symbols, e.g.:
    ex a = pow(sin(x), 2) - pow(cos(x), 2);
    ex b = sin(x) + cos(x);
    ex d;
    lst l;
    divide(a.to_rational(l), b.to_rational(l), d);
    cout << d.subs(l) << endl;
  will print "sin(x)-cos(x)"