- Added ISG_extensions.rb, which declares methods to extend existing
  Sketchup classes (Group for now).
- Added Controller::initialize_shape(group) which sets up basic shape,
  which is used for rule application.
- Added shape_UIDs so we can track each shape instance.
- Controller now initializes all ISG shapes upo initialization.
- Started developing create_rule method, but it is not yet functional.
- Added Controller::define_rule which defines rule composition.
- Added Controller::set_UID(entity, uid) which sets unique ID to
  entity specified (for now we use it on origin markers).
- Improved initialization of shapes and markers.
- Added comments.
- Added automatic initialization of existing Shape Rules.
- Improved initialization mechanism.