addWatch now outside inViewport
adjust debounce scroll check time
upgrade tests

We can't use innerHTML to insert test divs, IE < 10 does not like that

Use plain DOM instead.
+ higher wait after a scroll.

tl;dr; we wait 15ms to say "it is visible" so we should wait at least
15ms between 2 scrolls. Wait 30ms to be sure.

Scroll dependent testing is tricky, mastering it now ! :-)