- XS.subclass():
  - allow base class to be null to allow the creation of top base classes
  - set derived.subclass() to ease subclassing without the subclass() function
- pipelet.js:
  - use new subclass()
  - Controllet_Input becomes Controllet.Input, Controllet_Output becomes Controllet.Output
  - Union_Input becomes Union.Input and Union_Output becomes Union.Output
- Update documentation