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

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.