Fix: cancel all pending animation callbacks

If clear is called while the list is scrolling, any pending animation
callbacks were still pending causing the list to jump when they were
triggered.

LiteList.js: save off the animation frame id and cancel it on clear.

scroll.js: save off the tween animation frame id and cancel both it and
any pending tracker timeouts on reset.
Merge pull request #14 from davidcl64/fix_cancel_pending

Fix: cancel all pending animation callbacks on LiteList.clear()