Admin features

Added following functions to the pb.AdminNavigation

    pb.AdminNavigation.add(node)
    pb.AdminNavigation.addChild(parentId, node)

addChild can be called at any time even before the parent tree node exists, this is just in case someone will ever need to do something like this. The navigation will only be built once on startup so the overhead should be really low.

i also added an adminSample Plugin to show off the admin related features.
You'll also need a remove function to allow plugins to unregister themselves from the admin navigation.  We ran into this use case just the other day with our word press plugin.  It adds a sub-nav button to the plugin settings screen.  It did not remove that registration with AdminSubnavService.  Therefore, if the user uninstalls the plugin then reinstalls without restarting all PB processes the same callback would be registered again.  This causes angular to freak out because it does not have a unique set of items to work with.