Add explicit kwargs for plot_contours and plot_datapoints

I wanted to make a plot where I don't show contours, and I had to *look at the code* to figure out how to do that. Preposterous.

There's also an API change here: `smooth` has been renamed `plot_contours` to be more like `plot_datapoints`. But `smooth` will still work because, yea, backwards compatibility and all that.

I also committed heresy and reformatted  the main docstring to use the `numpy`-style docstring formatting. I think it's easier to read than having weird things like ':param' floating around...