Add support for multiple document managers (issue #79)

Initial attempt at addressing issue #79.

Note that I haven't done any new tests yet; all the existing ones pass except for this one, which is the same as in master for me:

```
1) Symfony\Cmf\Bundle\MenuBundle\Tests\WebTest\Admin\MenuNodeAdminTest\MenuAdminTest::testMenuShow
Failed asserting that 404 matches expected 200.

/var/www/symfony-cmf/MenuBundle/Tests/WebTest/Admin/MenuAdminTest.php:37
```

In order to test this, I think we need the test bundle to support multiple document managers in the functional tests. Same goes for RoutingBundle and BlockBundle.