Overlaping and nearby markers will "spider" out into a circle or spiral when zoom level is 12 or more (pretty zoomed in)
Overlapping markers have the popup disabled until they are popped out and "spiderfied", after that you can click on the individual markers and get the popup info as normal.
Click back on the map a second time to disable the spidering effect.
Current zoom level can be seen in the URL `map_display.php#13/XX.XXXXX/XXX.XXXX` `map_display.php#8/XX.XXXXX/XXX.XXXX` is zoom level 8, etc.
The "spiderfy" effect is currently constrained to zoom level 12 or greater.
This option may become configurable in the settings file if needed. 12 seems a good balance for now.

Fixes #19