iScroll support

There are many use-cases where we need not full-screen scrolling. Fo example tiny widget with twitter updates or comments.. whatever. In this case [iScroll](https://github.com/cubiq/iscroll) or other library used to scroll content through fixed size div (wrapper).

With this patch it is possible to just bind `wrapperTop` and `wrapperHeight` to let `ember-cloaking` do the job.

Demo -- http://jsbin.com/ACikukuCA/29/edit