use plt.subplots() in examples as much as possible

At the recent LBL Software Carpentry Workshop, it was pointed out that there's
an inconsistency within our documentation for how to create new figures with
subplots.

Indeed, most examples were using the old way, something like:

    fig = plt.figure()
    ax = plt.subplot(111) # or plt.add_subplot(111)

This patch changes a whole bunch of instances like the above to:

    fig, ax = plt.subplots()

We should strive to have a minimal amount of constants in our code,
especially unusual ones like `111`, which only make sense to Matlab
refugees.

I have left unchanged examples which were using axes keywords passed to
subplot() and add_subplot(), since those end up transforming things like:

    figure()
    subplot(111, axisbg='w')

to

    plt.subplots(subplot_kw=dict(axisbg='w'))

which isn't necessarily better.

I also did not touch most of the user_interfaces examples, since those did not
involve using plt, but instead explicitly imported Figure, and used the OO
approach on Figure instances.

Also updated instaces where the old "import pylab as p" convention was used to
use our standard "import matplotlib.pyplot as plt"

I have also updated some, but not all uses of subplot(121) etc, but I'm a bit
exhausted after doing all of these.
revert to OO interface (no pyplot) in OO example
more `subplots`, cleanup of old `import *` style
whats_new: a note about the changed examples
addressing PR feedback
Merge branch 'use-subplots' into v1.3.x
Fix pdf_use14corefonts test
Fix tests
Merge pull request #454 from mdboom/xdg_config

Use a subdirectory of $XDG_CONFIG_HOME instead of ~/.matplotlibrc on Linux
Merge pull request #2050 from pelson/colormap_api

Added the from_levels_and_colors function.
Fix PEP8 whitespace issue
Update logo_sidebar_horiz.png
backend gtk and gtk3: destroy figure save dialog after use
Merge pull request #2074 from efiring/gtk3save

backend gtk and gtk3: destroy figure save dialog after use; closes #2073
Merge remote-tracking branch 'upstream/v1.2.x' into v1.3.x
Merge pull request #2024 from mdboom/new_logo

Update homepage image
Merge pull request #2066 from GBillotey/oversized_doc_img

[DOC] Mathtext and matshow examples
Merge pull request #2076 from cgohlke/patch-1

Build the _windowing extension
Merge pull request #2077 from rblomberg/yahoo-csv-url-fix

changed URL to the current CSV API for yahoo finance
Fix bug in XFG_CONFIG handling