Support flow annotations for component props

[phpnode]

This PR adds support for using flow type annotations for class props instead of propTypes. Flow added support for this quite recently in: facebook/flow#850

The syntax looks like this:

type Person = {
  name: {
    firstName: string;
    lastName: string;
  },
  age: number;
};
type Props = {
  greeting: string;
  people: Person[];
};
class Hello extends React.Component {
  props: Props;

  render () {
    const names = [];
    for (let i = 0; i < this.props.people.length; i++) {
      names.push(this.props.people[i].name.firstName);
    }
    return <div>{this.props.greeting}, {names.join(', ')}</div>;
  }
}

[yannickcr]

Wow! This is really great!

Can you squash your commits ? Then I think we'll be good to go :)

[phpnode]

@yannickcr squashed

[yannickcr]

Merged, thanks!

[phpnode]

awesome, thanks!