init historian in ctor

This is closer to what I said, but it's lacking the core idea. Historian was been introduced to make mocking easier. The 2nd ctor would help that, though I don't see why `PlaceManagerImpl` (which is abstract) would provide a default implementation. I would give that role to DefaultPlaceManager. And since the latter has an injected ctor, you can get rid of the GWT.create().

```
DefaultPlaceManager extends PlaceManagerImpl {
 @Inject DefaultPlaceManager(EventBus eventBus, TokenFormatter tokenFormatter, DefaultHistorian historian) {
    super(eventBus, tokenFormatter, historian);
  }
}

PlaceManagerImpl {
  PlaceManagerImpl(EventBus eventBus, TokenFormatter tokenFormatter, Historian historian) {/*snip*/}
}
```