Add CLI with argument validation and defaults.
Add cluster utility.
Move defaults objects into lib/args.js
Extract lib/cluster/worker.js from cluster/index.js
Add stub tests for cluster master and worker.
Only call worker#listen() if cluster.isWorker
Extract lib/defaults.js out of lib/args.js

* Changes port default to 8000, matching app.js
* basePath should not have a default, as its meaning has changed since first implemented.
Add --cluster option to CLI, defaulting to single-threaded default server.
Don't filter args passed to workers.
Resolve route rootPaths from dirname of rootsFile, if it exists.
Cluster master constructor should not call anything based on config, do that in cli.js.
Logging during cluster events should be synchronous (console.error)
When worker restarted, clean up old pidfile.
Add cluster worker tests, with robustification.
Extract base class from cluster master and worker.
ComboWorker extends ComboBase.
ComboMaster extends ComboBase.
Add _maybeCallback utility method to ComboBase.
ComboBase#destroy() should remove all instance listeners.
Add rimraf devDependency.