Add a index.php server-side entry point. Also include a sprintf-ready version of claro/index.html. Template path resolution is determined by the required "template" request parameter.

We'll eventually remove the old index.html entry point as the server-side entry point saves us several requests:

 * A call to CreateSession.php
 * A call to fetch the ApplicationDefinition resource

Also, having a server-side entry point allows for users to add their own extra initialization or security checks before the fusion template is allowed to load.