WIP: virtual scroll

@robdmoore if you wanna test, here it is so far

Test in /test/html/list-fit.html

TODO:

- [x] unit tests
- [x] docs
- [x] grid
- [x] cleanup on $destroy
- [x] one more run of optimizations
- [x] errors on bad developer usage
- [x] allow user to input item size
- [x] fix visual bug while scrolling back up/left
- [x] fix transclusion bug while scrolling down