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

tests/queryset.py

@@ -5,8 +5,9 @@ import unittest
 import pymongo
 from datetime import datetime, timedelta
 
-from mongoengine.queryset import (QuerySet, MultipleObjectsReturned,
-                                  DoesNotExist, QueryFieldList)
+from mongoengine.queryset import (QuerySet, QuerySetManager,
+                                  MultipleObjectsReturned, DoesNotExist,
+                                  QueryFieldList)
 from mongoengine import *
 
 
@@ -1737,6 +1738,53 @@ class QuerySetTest(unittest.TestCase):
 
         Post.drop_collection()
 
+    def test_custom_querysets_set_manager_directly(self):
+        """Ensure that custom QuerySet classes may be used.
+        """
+
+        class CustomQuerySet(QuerySet):
+            def not_empty(self):
+                return len(self) > 0
+
+        class CustomQuerySetManager(QuerySetManager):
+            queryset_class = CustomQuerySet
+
+        class Post(Document):
+            objects = CustomQuerySetManager()
+
+        Post.drop_collection()
+
+        self.assertTrue(isinstance(Post.objects, CustomQuerySet))
+        self.assertFalse(Post.objects.not_empty())
+
+        Post().save()
+        self.assertTrue(Post.objects.not_empty())
+
+        Post.drop_collection()
+
+    def test_custom_querysets_managers_directly(self):
+        """Ensure that custom QuerySet classes may be used.
+        """
+
+        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()
+
+        Post.drop_collection()
+
+        Post().save()
+        Post(is_published=True).save()
+        self.assertEquals(Post.objects.count(), 2)
+        self.assertEquals(Post.published.count(), 1)
+
+        Post.drop_collection()
+
     def test_call_after_limits_set(self):
         """Ensure that re-filtering after slicing works
         """