Implement HasMenuCurrent and IsMenuCurrent for Nodes

Prior to this commit, `HasMenuCurrent` and `IsMenuCurrent` on `Node` always returned `false`.

This made it hard (if possible at all) to mark the currently selected menu item/group for non-Page content (home page, category pages etc.), i.e. for menus defined in the site configuration.

This commit provides an implementation of these two methods.

Notable design choices:

* These menu items have a loose coupling to the the resources they navigate to; the `Url` is the best common identificator. To facilitate a consistent matching, and to get it in line with the menu items connected to `Page`, relative Urls (Urls starting with '/') for menu items in the site configuration are converted to permaLinks using the same rules used for others’.
* `IsMenuCurrent` only looks at the children of the current node; this is in line with the implementation on `Page`.
* Due to this loose coupling, `IsMenuCurrent` have to search downards in the tree to make sure that the node is inside the current menu. This could have been made simpler if it could answer `yes` to any match of any menu item matching the current resource.

This commit also adds a set of unit tests for the menu system.

Fixes #367