BUG: enabling subplots works unexpectedly
Found 2 minor issues related to plotting with `subplots=True`.
1. If `DataFrame` only has single column, plot raises `TypeError`. It is because `_subplots` returns `Axes` instance, not array.
2. If `kind=scatter` or `kind=hexbin` is specified with `subplots=True`, additional blank axes is drawn.

I found 3rd minor issue and fixed:
- Calling `Series.plot` with `subplots=True` raises `AttributeError`
 seems reasonable, @TomAugspurger can you validate?
 This is good. @sinhrks do you want this one merged before your #6656?
 Yes, please. Later, I'll finish #6656.
 @jreback  this is OK, just a conflict in `release.rst`

Can I merge this from the command line with

```
git checkout -b sinhrks-scattersp2 master
git pull git://github.com/sinhrks/pandas.git scattersp2
```

fixing the conflict in `release.rst`, rebasing onto master, checking out master and merging this PR in, and pushing to upstream/master? I shouldn't force push to master right?
 correct, don't force push to master

exactly correct on your git-fu!
 OK thanks.

merged via 97c4a2e5e47bf00d1da5d56b7fbf862b2fc52819

Thank @sinhrks 
 @TomAugspurger maybe add this to the dev wiki somewhere? (how to merge a PR this way)
 https://github.com/pydata/pandas/wiki/Merging-Commits started there. 
 nice!
