Simplify Axes.scatter to better handle arguments

This PR simplifies the overloaded Axes.scatter method as follows:

• handle an s (size) array as well, seaborn 0.11.1 expects these to be acceptable as positional arguments,
• renamed c_sort{=True} to sortbycolor=False, to have a better name and have the same default action as matplotlib's Axes.scatter

@alurban @eagoetz, this probably has implications for the summary pages, which I think rely upon the default sorting by colour.