DOC : improved `extent` of `imshow` doc

closes #990
slight edits to docs as suggested by @mdboom
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
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
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
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