DOC : improved `extent` of `imshow` doc

closes #990
slight edits to docs as suggested by @mdboom
BUG : instantiate fall-back writer

fixes #2651

If the a string is passed into save as `writer` the code tries to
look up a writer with that name and create it.  If a writer with that
name is not in the registry, it is replaced with the name of the first
writer that _is_ in the registry, but not an instance of that type.
BUGFIX:  This change fixes #2475, where contour labels added manually
produce ugly spikes in the contours.  The problem was that screen
coordinates were being passed as data coordinates when the contour was
split to add room for the label.

Incorporate my fix and miili's #2806 change
Fixes a bug in font family lookup.  Any font in the font alias
list (e.g. font.sans-serif) would be considered a perfect match whenever
the selected font alias is the first in the font.family list.  This
broke the priority ordering of font names.
Add test
Use assert_equal
No need to cast to floats with future division
EXP : turn of clipping in spine example

Turn off clipping in the spine placement demos.

Closes #2795
TST : added test to exercise transform kwarg to `add_label_near`

This exercises with input in both display and data units.
ENH Added the origin option to `spy`

This option allows to place either the origin at the bottom of the axes, or at
the top.
Update polar.py to fix matplotlib issue 2541
Update polar.py simply to use %= notation
BLD : adjust min six version to 1.3
Use 'y#' instead of 's#' in write_rgba of the agg backend. Fixes #1891.
#2150 work in progress. Patch limits get updated even if width or height of a patch might be 0 (but not if both width and height are 0).
doc changed
#2150 wip - added change log
#2150 updating comments
remove .rect member (clashes with QWidget)

FigureCanvasQTAgg inherits from QWidget, which already has a 'rect'
property and getter.

Also, consolidate .rect and (boolean) .drawRect into a single _drawRect
member.  This is only used for the zoom rect and not accessed from other
classes (other than indirectly via the drawRectangle() function).