Axis Indexing Issue
Is the following difference between the `-` operator and the `sub` method a design decision?

``` python
import numpy as np
from pandas import DataFrame

x = DataFrame(randn(3, 2))
xm = x.mean(axis=0) # broadcast along the rows
xc1 = x - xm
xc2 = x.sub(xm, axis=1) # broadcast along rows with axis == 1??
assert np.array_equal(xc1, xc2)
```

This seems a bit unintuitive.
