Global search

see http://sonata-project.org/blog/2013/10/1/admin-bundle-global-search
requires SonataAdminBundle master version

currently fails on http://cmf.lo/app_dev.php/en/admin/search?q=CMF with:
```
An exception has been thrown during the rendering of a template ("No initial constraint found when dispatching whereOr - call where() first") in SonataAdminBundle:Core:search.html.twig at line 48. 
```

/cc @dantleech