Integrate into Rails scopes

[wollistik]

The descendants/descendants_of methods are instance/class methods, which cannot be use when using scopes. Find a way to allow things like

Node.where(foo: 'bar').descendants

instead of

Node.descendants_of(Node.where(foo: 'bar'))

[wollistik]

When this feature is implemented add a deprecation warning to switch over to new usage when an argument is passed to the class methods.

[wollistik]

Solution found: Using current_scope method of AR we can access the current relation when calling the method.