Version 0.0.4. Changed the way error messages are passed around.

  * The step functions only accept one argument, the done-callback function
  * If something is passed to the done-function it will get treated as an error
  Ø * The main callback function accepts an error object as its first argument, so it is possible to handle errors
  * No further processes will get started if a done function receives an error-argument

This break backwards compatability. If you have made something with operandi you should update your code.