DOC : improved `extent` of `imshow` doc

closes #990
slight edits to docs as suggested by @mdboom
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).
#2150 wip - added change log
#2150 updating comments
Re-Generate legend, through apply_callback/Apply

If there is a legend, it is re-generated when "Apply" is clicked.
See Issue #2934:
Might have side effects.
matplotlib#2934
tacaswell: "However this will clobber any legend that is not the auto-magically generated one so it should probably also get a tick box to enable/disable the updating of the legend."
Tick box not included.
allow slice and fancy indexing to only show some markers
show markers based on distance along line, split sequence behaviour into tuple and list/array
tightened up type checking. added some error handling.
Added a pylab example for markevery
wrote additional markevery tests in test_axes.py
added baseline images for markevery test in test_axes.py
pep8 fixes
added try except to check for valid iterable
reworked set_markevery docstring to bullet list