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.
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
(fix #2097) PGF: get fonts from fc-list, use builtin fonts for tests
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.
Don't close figure if context and apply_rcparams are both set. Fixes #2890.
#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).