Add bool as a mongo-safe type.

Note that bool is a subclass of int, so isinstance() made it work. Wow,
my confidence in isinstance(is really shook right now.

src/mongopersist/interfaces.py

@@ -23,7 +23,7 @@
 from bson import objectid, dbref
 
 MONGO_NATIVE_TYPES = (
-    int, float, unicode, datetime.datetime, types.NoneType,
+    bool, int, float, unicode, datetime.datetime, types.NoneType,
     objectid.ObjectId, dbref.DBRef)
 REFERENCE_SAFE_TYPES = (
     datetime.date, datetime.time, decimal.Decimal)