Google Map custom

@LarryMatte if you need to do that kind of thing one day, I added comments, so we can understand it and redo it later. Wasn't easy (thanks @spg)

![screen shot 2014-09-08 at 2 58 08 pm](https://cloud.githubusercontent.com/assets/2584073/4191020/5d67f1f6-378a-11e4-97a1-65a1b4fdc38d.png)

extract version to Maven constant; `${gwt-maps-api.version}`