AttributeRouting + Gravatar Helper
More Groundwork

* Added MvcSiteMap (still needs to be config'd)
* Added Login & Register forms (still need to be plumbed)
* Added a couple helpers, including an AjaxOnlyAttribute and a Script
registration extension to be used later.
* Added 403 error (used in AjaxOnlyAttribute) and config'd error
handling a bit more.