ENH: Added the level arguments to the series and frame math operations
such as sum, etc.

Awesome, thanks. I did the DataFrame docstring templating stuff I mentioned here and updated the return types in the Series methods

https://github.com/wesm/pandas/commit/e775d09ff5ad652090b84c6bb4658469a534e588

I'll chase down the groupby error separately
 fixed the bug in the above commit, turned out to be pretty simple
 I just noticed that the DataFrame methods don't handle the `axis == 1` case, I'll fix that
 Need to also test skipna=True vs. skipna=False
 ok, I think I've got everything sorted in the above commit. Maybe a little test coverage remaining
 I can write a couple more tests tomorrow if you'd like.
 No worries, already added the additional tests. Seeing if I can get the release ready or almost tonight
