Dependency Tree

We finally have a tree with help with DagreD3.

The code is a little messy. The plugin itself does nothing right now, everything happens inside the directive.
No node dragging so far - this will be solved through the token directive and won't be the responsibility of the tree it seems.

The exact design of that is not clear so far.

The reusability of the `dependencyTree` directive will be put to test soon, when we are going to build diff views.