Simple support for google maps infoBox-plugin.

I added a very simple check to see if the window has `options.boxClass` set and if the google-maps constructor-function InfoBox is loaded. If so the info-box-plugin is used to display this box instead of `new google.maps.InfoWindow`.

![screen shot 2014-01-30 at 9 59 28 pm](https://f.cloud.github.com/assets/583593/2047571/c499893a-8a23-11e3-970f-c346baf696a4.png)