Need to support React 16

[gimdongwoo]

React 16 has a lot of changes compared to 15, so we need to deal with it.

[bgribble]

I'm using react-chatview 0.2.5 vs. react 16.2.0 with basically no problems but one bad behavior that I can't pin down. When I switch from a large set of items to display down to a smaller set without unmounting the <ReactChatview />, under some circumstances there are leftover children from the previous dataset still in the DOM, but which do not appear in the component's props.children. Unmounting and remounting fixes this problem.

As usual this is probably my own damn fault somehow but I believe I've only seen it since we migrated to React 16 a couple of weeks ago. I'll try to get more information to make this a useful report.