PERF: mean much slower than sum for bool dtype
```
In [57]: # float dtype

In [58]: s = Series(rand(1000000))

In [59]: timeit s.mean()
1000 loops, best of 3: 1.03 ms per loop

In [60]: # int dtype

In [61]: s = Series(randint(100, size=1000000))

In [62]: timeit s.mean()
1000 loops, best of 3: 1.33 ms per loop

In [63]: # bool dtype

In [64]: s = Series(rand(1000000) > 0.5)

In [65]: timeit s.mean()
10 loops, best of 3: 26.5 ms per loop
```

i'll submit an issue to `bottleneck`, but just wanted to bring it up here in case we want to default to the non-bottleneck impl for bool dtype.
