rewritten ScrollScene to make trigger elements optional

The trigger element for a scene is now also optional. If none is passed
the scene will start, when the user starts scrolling (unless an offset
is set).
BIG CHANGE! not compatible to previous versions...
major changes

removed addNewScene (too confusing)
and instead unified usage of controller.addScene(scene) and
Scene.addTo(controller)
Also unified all options to be settable on the fly.
corrected a small mistake when verifying triggerHook setting.
removed set parent of scene because the parent is automatically set
now, when adding the scene to the controller.
Pin changes

Fixed issue https://github.com/johnpolacek/superscrollorama/issues/138
to allow for pinned elements that are positioned using percentage
values.