Add a "latest available release" information

Created by: galuel

Cesium should give the user information the latest available version on the software, and also on its official site.

It is possible to do it in javascript like here : http://geconomicus.glibre.org/latestreleases.html

So people with version 1.7 will be aware it exists a version 1.9 for instance, and so he can update knowing the information.