- optimized determinant calculations for small matrices (size <= 4)

(only 30 muls for size 4)
- rework the matrix inversion: now using cofactor technique for size<=3,
  so the ugly unrolling is only used for size 4 anymore, and even there
  I'm looking to get rid of it.

bench/benchmark.cpp

@@ -28,7 +28,7 @@ int main(int argc, char *argv[])
     asm("#begin");
     for(int a = 0; a < REPEAT; a++)
     {
-        m = Matrix<SCALAR,MATSIZE,MATSIZE>::ones() + 0.00005 * (m + m*m);
+        m = I + 0.00005 * (m + m*m);
     }
     asm("#end");
     cout << m << endl;