Merge pull request #1 from oholworthy/voronoi-overlay

Adding voronoi overlay of results. Copied from here: http://bl.ocks.org/...
Beer icon
5km radius, also anchor the beer icon to it's center (more or less)
A bit of a tidy up

Move the overlay variable out of the overlayVoronoi function, so it can
be referenced later when redrawing.

Same for the chiswick variable.
Attribution
More attribution
Beer icon attribution
cleanup again
overlay colours