Fixes #262, visibility attribute is not a "known" attribute and should not be filtered out of the node.
Fix described in #263 - null reference exception when the user logs off of their account when using Canonical, MetaRobots, or Title HTML helpers. Added a guard to check whether the current node is null to the templates.
Added naming functionality to FilteredSiteMapNodeVisibilityProvider so named instances of menus can have different visibility.
Added the Site.Master page to the MvcMusicStore project because it wasn't included. Removed the _ViewStart and _Layout pages because there were no files corresponding to those entries.