Implemented nextUpdate() for React testing

Per conversations with @petehunt in IRC, this PR adds a new `nextUpdate(component, callback)` method in ReactTestUtils.

This is a convenience method for adding an additional, one-time `componentDidUpdate` method to a component. It's useful for testing components that are updated asynchronously from outside data sources (such as components that fetch data through AJAX). For example, if you had a component that changed a value that was updated asynchronously, you might test in Mocha it with something like:

```js
it('updates the value of input.foo with the current state.value', function(done) {
  expect(1);

  ReactTestUtils.nextUpdate(component, function() {
    expect(component.getDOMNode().querySelector('input.foo').value).to.be(expected);
    done();  // (mocha-style async callback)
  });

  setTimeout(function() {
    component.setState({value: expected});
  }, 10);
});
```

Internally, this creates a new `chainedFunction` like a mixin would, but resets `componentDidUpdate` back to its initial value after the callback was called.