Highlight vertexes when no points

When `series.points.show` is false, currently a ring appears on highlight around where the point would have been.  This pull request changes that behavior to display a filled-in point on highlight.

Here's a jsfiddle of the current behavior: http://jsfiddle.net/trask/GrmYW/

Here's a jsfiddle using the patched jquery: http://jsfiddle.net/trask/xC2sF/