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.
API: raise on setops for + and - for Indexes (GH8226)
Provides a link to the list of encoding in Python

Better links

Added link to standard encodings in io html documentation

Removed link to Python2 documentations and left link to Python3 only
Merge pull request #8227 from jreback/index_api

API: raise on setops for + and - for Indexes (GH8226)
BUG: DatetimeIndex.asof matches partial dates
Merge pull request #8125 from dxshank/dev-update-read-csv-docs

DOC: Link to Python standard encodings from read_csv documentation
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
ENH: Add duplicated/drop_duplicates to Index
New "ambiguous" argument in tz_localize to enhance support for ways of resolving transition times ('infer', dot indicators, 'NaT', and 'raise')