replaced power operator with shift operator

Again, I don't know if the readability should be sacrificed for the (potential but negligible) performance benefit:

```
$ python3.4 -m timeit "128 ** 20"
100000000 loops, best of 3: 0.0197 usec per loop
$ python3.4 -m timeit "1 << (7 * 20)"
100000000 loops, best of 3: 0.0205 usec per loop
```