A quick hack to get the WebAgg backend working with `ipython --pylab` until proper IPython integration can be done.
added explicit 'zorder' kwarg to `Colection` and `LineCollection`.

removed class level zorder assignment.

It is not strictly necessary for LineCollection to take zorder as a
kwarg, but added to keep the default zorder the same
changed how `draw` handles alpha in `markerfacecolor` so that the set
value respected.
added guards to deal with 'none' case properly
added test for translucent markers
added change log entry for #1504 and #1505
documentation tweak
pep8 fixes
Merge pull request #1505 from tacaswell/issue1504

NF - Transparency for markers.
Merge pull request #1651 from mdboom/webagg/pylab_compatibility

WebAgg: pylab compatibility
Merge pull request #1654 from tacaswell/collection_zorder_kwarg

added explicit 'zorder' kwarg to `Colection` and `LineCollection`.