Fixed calculatePointRadius algorithm and altered offset for y labels

line.js
* Corrected calculatePointRadius.  Previously it wasn't correctly
  accounting for the fact only half of the stroke-width factors into
  the radius.
* Changed the y-offset used for point labels.  Previously the formula
  was using the half-width of the label, it has been changed to
   use the radius of the point.

Additional notes
* Removed padding from .elroi-point-label; it is no longer necessary
  due to the y-offset changes.
* Update tests related to calculatePointRadius.