Replace React.__spread with Object.assign

[DawidJanczak]

Addresses #70 React 0.15.1 works, but throws warnings when using __spread.

@jsdf you mentioned still supporting __spread, but it will be removed from React soon. Can we not just replace it with Object.assign?

[max-degterev]

Probably needs a warning that Object.assign polyfill is required

[DawidJanczak]

I'm a bit new to Node/npm so forgive me if I'm misunderstanding this, but React already includes that polyfill as its dependency. In fact even if we stick to React.__spread here it will eventually get resolved to Object.assign anyway https://github.com/facebook/react/blob/master/src/isomorphic/React.js#L36

[jsdf]

I don't think react's Object.assign polyfill will help here, as looking at the compiled react library, it looks like they are transforming their code to replace calls to Object.assign with calls to a Object.assign 'ponyfill', which doesn't touch the global Object. This means that people using coffee-react generated code which contains spreads will need to provide their own polyfill. I'll update the readme before cutting the next release.