Add exclusive popup for OpenLayers click popups.

This commit adds an option to enable exclusive popups. This means that when
you use click popups only one popup is visible. If you click on another marker
the other will close.
Add exclusive mode to "Show popups" dropdown.

Moved the exclusive popup mode for OpenLayers to the "show popups" dropdown menu.
Merge pull request #7 from dawnbreak/exclusivePopup

Add exclusive popup for OpenLayers click popups.