Test that Node is a function before using instanceof on it

We use React on a page that also uses the [Sarissa][1] library (as part of [RichFaces][2]). This library defines [its own `Node` object][3] if the browser doesn't already have one (as IE8 does not). This causes the `instanceOf` check in isNode.js to throw an error.

Reduced test: http://coonrod.org/react/react-0.9.0/examples/basic/

This is the "basic" example from React using the unmodified React 0.9.0. I've added the Sarissa library as well as es5-shim.js and es5-sham.js. If you view it in IE8 you will see something like this:

![screen shot 2014-02-25 at 6 13 28 pm](https://f.cloud.github.com/assets/58823/2265933/a2a8398a-9e8b-11e3-8b7c-1f9e0dbcc188.png)

and the timer does not time.

I believe you want to always use `instanceof` with a function anyway so it seems like a good thing to test when you don't know if you have a good Node or not.

[1]: http://dev.abiss.gr/sarissa/
[2]: http://www.jboss.org/richfaces
[3]: http://sarissa.cvs.sf.net/viewvc/sarissa/sarissa/src/main/js/gr/abiss/js/sarissa/sarissa.js?revision=1.31&view=markup#l69