Adds Overflow.scrollY(el) which makes an elment scrollable in iOS.

Normally, iOS shuts off overflow scrolling. Calling
Overflow.scrollY(el) will make the element scrollable via touch
events. There is no velocity scrolling, however.
Revove commented code.
Merge pull request #12 from WebFilings/add_overflow_handling_for_ios

Adds Overflow.scrollY(el) which makes an elment scrollable in iOS.