Added Custom Objects Managers

Managers can now be directly declared in a Document eg::

```python
    class CustomQuerySetManager(QuerySetManager):

        @staticmethod
        def get_queryset(doc_cls, queryset):
            return queryset(is_published=True)

    class Post(Document):
        is_published = BooleanField(default=False)
        published = CustomQuerySetManager()
```

Refactored the name of the `_manager_func` to `get_queryset` to mark it as
part the public API.  If declaring a Manager with a get_queryset method, it
should be a staticmethod, that accepts the document_class and the queryset.

Note - you can still use decorators in fact in the example below,
we effectively do the same thing as the first example and is much less verbose.

```python

    class Post(Document):
        is_published = BooleanField(default=False)

        @queryset_manager
        def published(doc_cls, queryset):
            return queryset(is_published=True)
```

Thanks to @theojulienne for the initial impetus and code sample #108

mongoengine/base.py

@@ -299,8 +299,10 @@ class TopLevelDocumentMetaclass(DocumentMetaclass):
         new_class = super_new(cls, name, bases, attrs)
 
         # Provide a default queryset unless one has been manually provided
-        if not hasattr(new_class, 'objects'):
-            new_class.objects = QuerySetManager()
+        manager = attrs.get('objects', QuerySetManager())
+        if hasattr(manager, 'queryset_class'):
+            meta['queryset_class'] = manager.queryset_class
+        new_class.objects = manager
 
         user_indexes = [QuerySet._build_index_spec(new_class, spec)
                         for spec in meta['indexes']] + base_indexes