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.
pep8 fixes
CHANGELOG, api_changes.rst, whats_new.rst
Merge branch 'slice-and-fancy-indexing-for-markevery'

Conflicts:
	CHANGELOG
	doc/api/api_changes.rst
	doc/users/whats_new.rst