Ehancement #2365 : Add markup root directly in ReactMount cache

@syranide Could you check this ?
I wanted to add checking on node parameter because we need a HTML element, but I'm not sure if this is the best way of doing this.
Also, if this okay, I might write some test for ReactMount.cacheNodeById.
It's always good to verify that the cache actually updated correctly :)
`dangerouslyReplaceNodeWithMarkup` does not currently return the new node.

https://github.com/facebook/react/blob/master/src/browser/ui/dom/Danger.js#L159