A few enhancements and upgraded to work with latest version of Ember

(I only just spotted that you've already got a branch in which you've started making the upgrade to the latest version of ember.js. Oh well. I've made some other changes as well.)

See below for summary of my changes. No worries, if you don't like them: I won't be offended. 

* updated example to work with latest release build of ember.js (v1.2.0), the latest version of handlebars.js (v1.3.0) and jquery.js (v1.10.2)
* both horizontal and vertical infinite scrolling enabled
* scrolling now works within a container viewport (which can be any html element), so infinite scrolling can easily be made to operate on part of the page (if required) rather than having to use the entire browser window as the viewport.
* edge case fix: infinite scrolling also works when top/bottom margins are applied to the scroll items. See commit f845179 for full explanation of this annoying little bug).
* README re-drafted to reflect above changes
* minimum styling recommendations added to README and demo to make it easier/quicker to get things working as expected