The singleton service that represents the top level router was called
`$router` but this is confusing since there are actually lots of routers,
which depend upon where you are in the DOM. This is similar to the situation
with scopes.
This commit clarifies this singleton by renaming it to `$rootRouter`.
BREAKING CHANGE:
The `$router` injectable service has been renamed to `$rootRouter`
Peter Bacon Darwin
Brian Ford
Victor Berchet