Coloring of conflicting cells based on user values

Method added to render to show conflicting row/col/block. More
complicated than anticipated, as all conflicts (created by multiple
different user-input cell values) need to be kept track of.  These
conflicts are now stored in the JQuery data attribute on the DOM nodes
themselves.

The render.neighborConflict method is now called by SUD anytime the user
updates a cell value.

Still needs some refinement, namely:
* Some way to clear conflicts based on two user values, once previous
  is removed. (e.g. user places a '3', no conflict, but then places
  another '3' in the same block, creating a conflict.  This works fine,
  but removing the first '3' will not remove the conflict associated with
  the second value.) Keeping a list of conflicts and recalculating them
  each time could fix if it's not too slow.
* The source of the conflict should have a unique styling to allow user
  to pick it out more easily.