Add support for Ajax node list

I've added a `tree` action to `NodesController`. This action responds by rendering a js.erb view that you'll find in [dradis-theme_snowcrash](https://github.com/dradis/dradis-theme_snowcrash/pull/1). This view is responsible for building the node list.
Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.