Supports custom events in addition to custom attributes:

• Most events are generated on the fly as needed. Whenever an unrecognized event is discovered, it generates a registration for it.
• This means most of the big lists of event names can go away, but I'm leaning on the top level event constants more (though I wonder if that could go away too)
• Testing custom events is sort of a drag, so I made ReactTestUtils.Simulate into a function, which accepts a custom event name

    it('simulated custom events capture', () => {
      var callback = jest.fn();

      var el = ReactTestUtils.renderIntoDocument((
        <div onCustomCapture={() => callback('capture')}>
          <button onCustom={() => callback('bubble')} />
        </div>
      ));

      ReactTestUtils.Simulate('custom', el.querySelector('button'));

      var sequence = callback.mock.calls.map(args => args[0]);

      expect(sequence).toEqual(['capture', 'bubble']);
    });