Explain that matplotlib must be built before the HTML documentation

Attempting to generate the HTML documentation right after cloning the
repository results in a cryptic error message as one of the required
files, mpl-data/matplotlibrc, is created when matplotlib is built. It
seems that other people have come across this problem in the past [1],
so mentioning it here might be of help to newcomers.

[1] http://sourceforge.net/mailarchive/message.php?msg_id=21478615
Add numpydoc to the list of extensions.
Added the autosummary extension module for sphinx
ENH Made the docutils tables generated with the numpydoc in the documentation a little prettier
Fix USE_FONTCONFIG == True mode.
fix "alpha" kwarg in errorbar plot

- this patch was rightly proposed by Hans Dembinski on the matplotlib
  devel mailing list but never incorporated it seems
changing errorbar test to cover "alpha" kwarg
Merge pull request #1675 from megies/errorbar_alpha

fix "alpha" kwarg in errorbar plot
Fix overwriting suptitle
Add suptitle overwrite test
Update test_pickle baseline image
Merge pull request #1663 from dmcdougall/fix_suptitle

Fix suptitle
Merge branch 'v1.2.x'

Conflicts:
	lib/matplotlib/tests/baseline_images/test_axes/errorbar_mixed.pdf
	lib/matplotlib/tests/baseline_images/test_axes/errorbar_mixed.png
Show custom error message if we don't have a matplotlibrc file

Even if the README.txt file explicitly mentions that matplotlib must
have been built before the documentation can be generated, a better
error message will be of help to those developers who happen not to
read it.
Show custom error message if we don't have a matplotlib installation
Mention that matplotlib must be installed before building the docs
Revert "When using nearest neighbor interpolation and not actually scaling, just copy the data.  (Note we don't do this for other kinds of interpolation, because the filtering on those interpolations could have an effect even when not actually scaling.)"

This reverts commit 1c9f9b5f6cb3b308df95e56f7230d71963837812.
Partial revert of 8c25664fcfcb9c2e288607ff86d0a1957248284f

This removes the flipping done in Python and keeps the regression tests.
Removed numpydoc
Added documentation on numpydoc