API: Treat 1D vectors as columns in some cases for arith/cmp ops
Definitely needs discussion.

There are a bunch of GH issues (will track down the list in a bit) that boil down to users expecting DataFrame+(Series|ndarray|listlike) ops to operate column-by-column.  This PR implements that behavior for a subset of cases.  In particular:

- `other` is a `Series`: indexes match exactly (and columns do not)
- `other` is a list/tuple/np.ndarray: length matches df.index (and not df.columns)

This means that things like `df == df['A']` will now work.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
