Don't wait for requestAnimationFrame to boostrap the initial view.

<cr-view> was using this.async() without passing a timeout
value which makes polymer use requestAnimationFrame.
Unfortuntely this also meant that in background tabs we
would never bootstrap the tab since RAF never fires if
the tab is not visible.

Instead lets just use setTimeout(0).

I also removed the MutationObserver for allowing dynamic
<cr-route> insertion and deletion. We don't need that kind
of dynamicism yet and it just adds complexity.

Fixes #41.