ENH: allow for setting a lists of labels with FacetGrid.set_axis_labels

This allows for easily setting different x or y axis labels to different
facets, without needing to use the matplotlib interface.

It does not break the API, aside from renaming the single argument
to `set_{x, y}labels` from "label" to "labels".

I also added a unit test for this method since I noticed it didn't have any
direct test coverage.