Make 'rstride', 'cstride' default values smarter.

This commit is a minor revision that may reduce headaches for new users. 

Previously, the default behavior for Axes3D.plot_surface resulted in very confusing plots for data of size less than 10X10. Additionally, inadvertently plotting very large matrices resulted in too many polygons being drawn, such that the lines along their edges obscure the polygons themselves.

This commit changes default behavior to draw as many polygons as possible not exceeding 100 to a row or column.