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
documentation tweak
Merge pull request #1654 from tacaswell/collection_zorder_kwarg

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