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 can use:
```
pb.utils.deepMerge(AdminNavigation.childrenAdditions, {});
```
to avoid having to declare another dependency. You can also use:
```
pb.utils.clone(AdminNavigation.childrenAdditions);
```
Not sure which one has better performance.