Update HoverData to include pixel positions.

Now includes array of Points corresponding with the elements being
hovered over.

Preparation for adding crosshairs.
Update scatter hover quicktest to use pixel position.
Properly account for pixel positions of horizontal bars, fix hover bug

CurrentHoverData needs to be set right away, so that onHoverOver() and
onHoverOut() callbacks can access it.
Pass in index to accessors.
Merge pull request #1269 from palantir/hover-interaction-pixel

Update HoverData to include pixel positions.