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.
appended newline to DataFrame.info() output
BUG: DatetimeIndex.asof matches partial dates
DOC: v0.15.0 for GH8226
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: created advanced indexing section
DOC: add internals section (GH4082)
Merge pull request #8204 from jreback/doc

DOC: reorg indexing docs
Merge pull request #8114 from BorisVerk/master

BUG: fixed df.info() output missing new line
BUG: plot methods modified rcParams
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.
Merge pull request #8247 from kyleam/fix-nonzero-docstring

DOC: Correct Series.nonzero docstring
Merge pull request #8249 from TomAugspurger/color-cycle

BUG: plot methods modified rcParams
DOC: Add note about bugfix
DOC: Fix conflict