Add a rake task to run rspec with a single file.
Tweak comment/documentation for app.rb
Commander::Server is powered by WEBrick

via the Rack::Handler::WEBrick rack-adapter.

Running atop a threaded http server has a few benefits:
* webrick is in the stdlib (so no platform specific compilation)
* not limited to using gems that are EM friendly
* easier to manage concurrency (EM lacks synchronization primitives).
  This is especially important for the apply() step of any txn, where
  we touch systemwide files.