Registering Cesium on node.js npm

This PR is to start a discussion about registering a subset of Cesium on npm so it can be brought in with a simple...

    var Cesium = require('cesium');

This could be useful for e.g. writing simple node.js scripts that generate CZML.  You should be able to test this by running the following commands from the Cesium root dir...

    npm install
    node node-test.js

Having node support for Cesium in this form would require ongoing maintenance of index.js and package.json along with updating npm on each Cesium release.  Presumably this would become part of the Cesium release procedure.  The current index.js supports pretty much all of Cesium Core with a few modules removed that depend on window.