Added ['style']['markersize'] to utils.get_marker_style.

For renderers that want high-level information, 'markersize' would be a
useful parameter. Otherwise, if renderers want a simple size in points,
it has to use the affine transform and the figure's dpi to figure it
out.
Merge pull request #12 from theengineear/master

Added ['style']['markersize'] to utils.get_marker_style.