Very simple way of implementing custom info-windows

Don't know if this is appropriate, as this is my first pull-request to an open-source project on git, but I added a very simple check (there probably lots of room for improvement here) to see if the google-maps function `InfoBox` is loaded. If so, `new Infobox` is used for all info boxes instead of `new google.maps.InfoWindow`. Boxes can then be styled via the `.infoBox`-class.