Hierarchical data management with Immutable.js

At the moment, immu-tree is at an early work-in-progress stage.

I'll be doing more in-depth documentation of the API once development is further along, but here's a rundown of what is currently functional:

Currently, the constructor is working similar to the 'stratify' method from d3-hierarchy(parses tree from array with id & parentId properties.

The constructor uses getters for properties representing collections of nodes including ancestors, parent, children, descendants, and leaves. Each of those properties returns a Map or OrderedMap containing instances of the immuTree representing each node.

More info and functionality to come!