Add Cesium version b17

@jimaek As you suggested, here's a pull request for [Cesium](AnalyticalGraphicsInc/cesium).  The Cesium community plans to put all new releases moving forward onto jsdelivr, so expect a pull request from us on the 1st or 2nd of every month.

I think I got the format and structure of everything correct, let me know if I did something wrong.

Thanks.