Make PlaceManagerImpl use Historian instead of History

Uses `Historian` interface in `PlaceManagerImpl` instead of `History`.

It will be impossible to replace the GWT History implementation in 2.7.  By switching to Historian GWTP users can replace the `DefaultHistorian` implementation with another eg. to support PushState.

This does have a minor impact on behavior.  Before when you called `revealCurrentPlace()` History would refire its state.  Now it won't.