Route repository manager

Following on from [PR #97](https://github.com/symfony-cmf/RoutingBundle/pull/97), I've made a similar change to ContentRepository:

* Constructor now takes the `ManagerRegistry` instead of `DocumentManager`
* New `setManagerName()` method changes which manager to look for in the registry
* Added unit tests (refactored to reduce repetition with `RouteProviderTest`)

As before... I haven't attempted functional tests, unless there's a consensus that requiring two workspaces to run the test suite is a good idea.