Add getUriContextCollection($document) method to manager

[dantleech]

Currently is is necessary to instantiate the UriContextCollection before using the RoutingAuto system:

$uriContextCollection = new UriContextCollection($document);
$arm->buildUriContextCollection($uriContextCollection);

foreach ($uriContextCollection->getUriContexts() as $context) {
    $autoRoute = $uriContext->getAutoRoute();
    // save auto route
}

But this causes potential BC breaks in the future as the URIContextCollection class is going to
be present in third-party code. It should be contained within the manager.