Add an example app to serve CZML over HTTP.

This is meant to be an example of the simplest case of serving some actual CZML content over HTTP, and displaying it on a Cesium globe.  It is designed with an intention for source-code readability more so than for functionality, but it is a complete working client/server Cesium application.
The data source can be added to the viewer before the data has loaded from `loadUrl`, removing the need for the `then` callback.