Ian/nav

@nicolashery dont merge just yet waiting for ui and product review
Good rule-of-thumb: if in a React app you feel the need to "touch the DOM" directly, there is probably something not right :) (unless you have a good reason to do so, for example setting up a D3.js component or other non-React stuff)

Here, there are a couple ways you could avoid this. One of them would be passing a `showingDropdown` prop to the Navbar component, but you might need to move the "showingDropdown" state up a level to App, which feels a bit awkward. Although calling methods on child components through "refs" shouldn't be over-used, it seems like a good compromise here. Something like:

```javascript
// App.js
render: function() {
  <div className="App" onClick={this.hideNavbarDropdown}>
  // ...
},

hideNavbarDropdown: function() {
  var navbar = this.refs.navbar;
  if (navbar) {
    navbar.hideDropdown();
  }
} 
```

```javascript
// Navbar.js
hideDropdown: function()  {
  if (this.state.showDropdown) {
    this.setState({showDropdown: false});
  }
}
```

Happy to chat more if you have any questions, or if it doesn't work out.