Vaguely related to issue #1622. Changed the location of the default zorder of Collection and LineCollection from a class level-member, to explicitly being a keyword argument.
I think this makes it clearer, but isn't strictly necessary, because if zorder is included as a kwarg, it will get set by the self.update anyway.
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
This line should probably be more along the lines of:
```The zorder of the LineCollection. Default is 2."""
Good stuff @tacaswell - I'm amazed we have come so far without zorder for Collections!
Thanks @tacaswell, merging.
(The Travis failures are duds)