[refactor] 'build-kit/taskrunner' module

[kixxauth]

The taskrunner module was hastily built to bootstrap the Kake project and therefore is not very robust.

Now

• Do error checking on the .append() method.
• Abstract the event emitter functionality or use an external notification emitter module (Another event emitter module is under construction elsewhere).
  • Event handlers may be bound after the .run() method and will still be fired OR the .run() method could return a promise.
  • Consider the possibility of running tasks in a future turn of the event loop and do some performance tests to see how much it would slow a large build script.
  • If event binding is used to monitor the 'done' and 'error' events, provide a method for unbinding the handlers.