[changed] activeRoute can render with props and children.

Potentially fixes #59.

`{this.props.activeRoute}` is now `{this.props.activeRoute(props, children)}`.
don't we need to `route.props.handler.apply(null, [mergeProperties(props, addedProps)].concat(children))` to support `this.props.activeRoute({}, child, child, child)` ?