Adds sunburst.js
Adds rubric:descent rake task

`rubric:descent` generates hierarchy.json — a tree-like structure that encodes the hierarchy of Ruby core classes.
Generates hierarchy.json
Adds rubric:ancestry rake task

`rubric:ancestry` takes data from `classify.yml` and generates `ancestry.json` — a JSON of direct_ancestry relations for each Ruby core class to be used by D3's edge bundling visualizations.
Generates ancestry.json
Adds edge_bundling.js
Adds CSS for edge bundling
Fixes misaligned links in diagram

Moves `text-align: center` rule out of `body` as it pushes the whole links diagram off to the right rather than being positioned correctly on the grid.
Adds radial edge bundling style & script
Renames files

`edge_bundling.js`        => `treemap.js`
`bundling.css`            => `treemap.css`
`radial_edge_bundling.js` => `edge_bundling.js`
`radial_bundling.css`     => `bundling.css`
Appends svg element to #viz instead of body
Adds visual switching buttons to layout

Adds four buttons (chord layout, sunburst, treemap, edge bundling) for dynamically changing the SVG diagram displayed and the associated header.
Gets dynamic visual switching working

This took way more time than it should have and probably can be refactored significantly, but at least this finally works.
Updates README

Adds a section about the data set and sections briefly explaining the four current visualizations.