fixed: Mouswheel over pinned element bug

as documented in these issues:
https://github.com/janpaepke/ScrollMagic/issues/34
https://github.com/janpaepke/ScrollMagic/issues/50
https://github.com/janpaepke/ScrollMagic/issues/82
https://github.com/janpaepke/ScrollMagic/issues/139
https://github.com/janpaepke/ScrollMagic/issues/140