Ajax lazyload for node list

The toggle icon now points to `NodesController#tree` action. This action is called after the `slideDown` animation has completed and renders a js.erb view that builds the node list, 

While at it, I've moved the node-selecting logic we had in the view to the model and controllers of Dradis (to keep everything in its right place) and took advantage of `acts_as_tree`'s native class and instance methods (`roots`, `ancestors`, `children`, etc) to traverse the tree.

I also took care of making sure that the current node's tree is expanded when viewing `NodesController#show` (not sure we had that already).