Id not being passed to the onChange function

Hi @davidtheclark! The library is great, thank you for open sourcing it!

I have been running into one issue. I was trying to replicate the stateless demo, and can get the tabs to switch. But the function being passed as the `onChange` prop is not being passed the `id` on `tabChange`. I imagine it is a n00b mistake, but any pointers would me much appreciated.

Here is what my render function works like -

``` javascript
return (

{renderTabHeadings()}

);
```

The `handleTabChange` function is not getting the `tabId` parameter. This is what that looks like:

``` javascript
handleTabChange = (newActiveTabId) => {
//TODO: change router here when updated
console.log(newActiveTabId); //is always undefined :(
};
```

Thank you!