Prevent swipeLeft or swipeRight too easy to trigger when scrolling body up and down

Due to [another swipe bug](https://github.com/madrobby/zepto/issues/622) filed in the issue tracker, this pull request is based on 1.0rc1.

Basically, the touch event is based on the position of the body. I need to subtract `scrollTop` and `scrollLeft` of body to get the actual touch position.