Fix streamplot for non-square grids.

[tonysyu]

• Order of x/y was switched.
• Additional cleanup (remove unused import and old comment, improve docstring).

[jswhit]

I just noticed that the patches for the arrows on the streamlines are not returned - so you can't remove them from the axes instance or alter their properties. Perhaps a streamline collection object could be created (and returned) that includes both the streamline line collection and the arrow patch collection.

[tonysyu]

@jswhit Yeah, the plan was to add this at some point, but it fell off my radar. Thanks for the reminder I just submitted a PR (#803) to return an arrow patch collection. I think creating a custom streamline collection object is a bit too much (work); instead, I just return separate collections for the streamlines and arrow patches.