Added better handling for React DOM nodes

Jade allows for arbitrary nodes whereas react has a defined subset. 

This patch validates the tag while adding the ability to pass react classes for tag substitution. 

* If a local is passed that matches a tagName and `react.isValidClass(locals[name])` it will use the class.
* If the tag name exists in react.DOM it will use that tag
* else it will convert the tag to a react.DOM.div

**See Tests**

My biggest question is this the best place to put this? My other thought was modifying jade.runtime and attaching the helper function there. 

Thoughts?

P.S. @ForbesLindesay great job with mixins!