[added] RouteHandler `activeRoute`

Most the time people ask about getting at route info, they just want to know the name of the active child route.

Today you can do that like this:

```js
render: function() {
  var child = this.props.activeRouteHandler();
  var name = child.name;
  return  <div>{child}</div>;
}
```

This is useful for some sidebar implementations (we have a scoped page search that does this too), and I'm sure many other things.

Might as well just give them the active route so the code becomes:

```js
render: function() {
  var name = this.props.activeRoute.name;
  return  <div><this.props.activeRouteHandler/></div>;
}
```