Rename `HelloWorld.html` to `index.html`

Also add Chrome Frame tag to Hello World so it works in IE if GCF is installed.

This change was largely a result of some end-user usability feedback in a recent Cesium hackathon.  Currently unzipping the Cesium release zip into a server and browsing to it currently yields something like the below.

![image](https://f.cloud.github.com/assets/1495196/371235/4991de6a-a322-11e2-83e8-9486df0b436f.png)

When we would want it to run Hello World instead.