Problems from swapping components

We're running into problems because we're trying to replace one with another within a single update. The new 's componentWillMount triggers before the old one's componentWillUnmount, resulting in a spurious "Only a single Gateway can be rendered at a time into a GatewayDest" warning, followed by the older gateway's componentWillUnmount removing the new gateway's children (because GatewayRegistry.removeChild doesn't care _whose_ children it's removing).

One solution is to add or restore support for multiple children (see #15 or [briteback/react-gateway](https://github.com/briteback/react-gateway)).