Fix for issue #94 - allow RouteProvider document manager change

As discussed in [issue #94](https://github.com/symfony-cmf/RoutingBundle/issues/94):

* Moved RouteProvider constructor argument out to new method `setDocumentManager()`
* I couldn't find a unit test for RouteProvider so I've started one (not complete though...)

Perhaps there should also be a functional test for this, however do we want to require anyone who runs the tests to create a second workspace?

Note: this will probably break anyone's code if they created their own service for the RouteProvider instead of using the one provided.