Removed CollectionManager, moved work to QuerySet

As CollectionManager has been replaced with QuerySet and
QuerySetManager, collection.py has been renamed queryset.py.

mongoengine/base.py

@@ -1,4 +1,4 @@
-from collection import CollectionManager
+from queryset import QuerySetManager
 
 import pymongo
 
@@ -153,7 +153,7 @@ class TopLevelDocumentMetaclass(DocumentMetaclass):
         # Set up collection manager, needs the class to have fields so use
         # DocumentMetaclass before instantiating CollectionManager object
         new_class = super_new(cls, name, bases, attrs)
-        new_class.objects = CollectionManager(new_class)
+        new_class.objects = QuerySetManager(new_class)
 
         return new_class