Changed google URLs starting with a hard coded http: to begin with // instead.

This way the protocol is dynamically chosen depending on the protocol of the site (http or https).
See RFC1808 and stackoverflow.com/questions/6785442/browser-support-for-urls-beginning-with-double-slash
Otherwise some browsers (like Chrome) would deny loading the map via http if the site itself uses https.

This should have no effect on existing http sites but fixing the issue with https sites.

By the way: nice project, well done!