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
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
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
tidy up
pulled the markevery logic out of the main draw function.