add ReactSubscription class to easily subscribe/unsubscribe react components in lyfecycle methods

[vitalets]

Before:

class Button extends React.Component {
  constructor() {
    super();
    this.subscription = new Channel.Subscription([
      {channel: onNewData, listener: this.handleNewData.bind(this)}
    ]);
  }
  componentDidMount() {
    this.subscription.on();
  }
  componentWillUnmount() {
    this.subscription.off();
  }
}

After:

class Button extends React.Component {
  constructor() {
    super();
    new Channel.ReactSubscription(this, [
      {channel: onNewData, listener: this.handleNewData.bind(this)}
    ]);
  }
}