Underlying constructor access

This is to support js interop where specific props are expecting a pure react constructor.  There is obviously a more elegant way to do this, but this is useful enough for me.

Below is an example of the interface I am exposing internally, specifically this becomes useful for passing the pure constructor to the handler prop.

```javascript
var Route = require('react-router').Route;

React.renderComponent((
  <Route handler={App}>
    <Route name="about" handler={About}/>
    <Route name="users" handler={Users}>
      <Route name="user" path="/user/:userId" handler={User}/>
    </Route>
  </Route>
), document.body);
```