Stagger element creation and rendering

These changes staggers the element creation and animations (also drops the animations after a configurable cutoff). It improves the painting performance mentioned in issue #24.

Here is the profile for the same scenario (as #24) with the updates. Painting time is cut by half. 

In our actual use case painting time (to stack view) is mere 64ms as Andy made other optimizations (like showing 3 items per stack and hiding all others etc).

![screen shot 2014-11-12 at 4 55 03 pm](https://cloud.githubusercontent.com/assets/8048953/5019322/b5db675c-6a8c-11e4-85f4-a7e995020a2e.png)