Added __call to the DocumentRepository for magic finders.

The use of magic finders, e.g. `$repo->findOneByNickname('romanb')`, is discussed in the [CouchDb ODM documentation](http://docs.doctrine-project.org/projects/doctrine-couchdb/en/latest/reference/working-with-objects.html#by-simple-conditions), but it seems it was never implemented.

I adapted this implementation from the [Doctrine 2 ORM](https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/EntityRepository.php). The only major change is using `call_user_func_array` to finally call the `find(One)By` method with the full arguments list rather than using a switch for each number of possible arguments.