replaced power operator with shift operator

This doesn't make a performance difference on my machine either:

```
$  python3.4 -m timeit "(1 << 20) - 1"
100000000 loops, best of 3: 0.0196 usec per loop
$ python3.4 -m timeit "(2 ** 20) - 1"
100000000 loops, best of 3: 0.0198 usec per loop
```