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: interpolate with no nans and limit
BUG: Bug in putting a PeriodIndex into a Series would convert to int64 dtype, rather than object of Periods (GH7932)
Merge pull request #8235 from TomAugspurger/interp-limit-nonan

BUG: interpolate with no nans and limit
BUG: DataFrame.to_string ignores col_space when header=False

Add test case for when header=False

Make col_space the minimum width of each col

Update release notes
Merge pull request #8237 from jreback/pi

BUG: Bug in putting a PeriodIndex into a Series would convert to int64
BUG: fix groupby crash on duplicated columns (GH7511)
Fix creation of database indexes (GH8083)

Tests fix

Fixes

Remove _executefunc

Fixed tests to work with Python3

Fix merge

Index test simplify

Test fix
Merge pull request #8083 from artemyk/to_sql_create_indexes

BUG: When creating table, db indexes should be created from DataFrame indexes
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)
appended newline to DataFrame.info() output
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
DOC: created advanced indexing section
DOC: add internals section (GH4082)
Merge pull request #8204 from jreback/doc

DOC: reorg indexing docs