Revised crosshair mouse move events

[andredumas]

Now that #20 is in place, #8 should be revised to emit events based on primary scales rather than configured annotations, as annotations may not be always specified but scales will be.

Rather than:

[
  [xAnnotation0.axis().scale().invert(x)],
  [yAnnotation0.axis().scale().invert(y), 
    yAnnotation1.axis().scale().invert(y), 
    yAnnotationN.axis().scale().invert(y)
  ]
]

Will be:

[crosshair.xScale().invert(x), crosshair.yScale().invert(y)]