API: support `c` and `colormap` args for DataFrame.plot with kind='scatter'

`matplotlib.pyplot.scatter` supports the argument `c` for setting the color of
each point. This patch lets you easily set it by giving a column name (currently
you need to supply an ndarray to make it work, since pandas doesn't use it):

	df.plot('x', 'y', c='z', kind='scatter')

vs

	df.plot('x', 'y', c=df['z'].values, kind='scatter')

While I was at it, I noticed that `kind='scatter'` did not support the `colormap`
argument that some of the other methods support (notably `kind='hexbin'`). So
I added it, too.

This change should be almost entirely backwards compatible, unless folks are naming
columns in their data frame valid matplotlib colors and using the same color name
for the `c` argument.

A colorbar will also be added automatically if relevant.
BUG: DatetimeIndex.asof matches partial dates
Merge pull request #7780 from shoyer/plot-frame-scatter

API: support `c` and `colormap` args for DataFrame.plot with kind='scatter'
Merge pull request #8246 from TomAugspurger/datetime-asof

BUG: DatetimeIndex.asof matches partial dates
DOC: Correct and expand Series.nonzero docstring

- Mention that a tuple of indices is returned and explain why.
- Instead of numpy.ndarray.nonzero, point to numpy.nonzero, which has
  the full documentation.
- Add example.