Change output on tick, not bound value

If this is used like `{{fromNow valueBinding="createdAt"}}` as in the example this will cause the original `createdAt` to be changed (two-directional binding) because of `notifyPropertChange('value')`.

More info on http://santtu.iki.fi/2013/12/11/gotcha-beware-of-bidir-traffic/.
Merge pull request #2 from santtu/patch-1

Change output on tick, not bound value