Fix for map reset/zoom-out / `xlink:title` tooltips on d3.js map-markers

[nfreear]

Hi Martin,

Thank you for getting on with pull #56 - much appreciated, as I was buried in my work.

The highlights of this quite small follow-up request are:

• 40 - a proper fix for map reset/zoom-out for both Leaflet & d3.js based maps that doesn't require a page refresh;

• 57 - xlink:title tool-tips for markers (SVG links) on the d3.js based map(s);

You'll note that it doesn't include fixes for #49 / #53, which are on the CR53-js branch - it needs debugging... ;).

Thanks,

Nick

[mhawksey]

Thanks for pushing on with this