Refactor: modularize

Refactor code from a one-file script to a series of classes that will encapsulate all the work.

* [ ] `Ruster::Node`
* [ ] `Ruster::Cluster`
* [ ] Test `Ruster::Node` functionality
* [ ] Test `Ruster::Cluster` functionality
* [ ] Refactor `bin/ruster` to use new classes
* [ ] Add documentation