my_power is really slow.

[dharhas]

The my_power implementation is really slow, I think we should replace it with something faster.

`In [3]: from sprint_tutorial.compute import my_power

In [4]: %timeit my_power(2.5, 8)
The slowest run took 4.27 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 1.62 µs per loop`