Added test inheriting document from another file works

Closes #28

tests/fixtures.py

@@ -0,0 +1,26 @@
+from datetime import datetime
+import pymongo
+
+from mongoengine import *
+from mongoengine.base import BaseField
+from mongoengine.connection import _get_db
+
+
+class PickleEmbedded(EmbeddedDocument):
+    date = DateTimeField(default=datetime.now)
+
+
+class PickleTest(Document):
+    number = IntField()
+    string = StringField(choices=(('One', '1'), ('Two', '2')))
+    embedded = EmbeddedDocumentField(PickleEmbedded)
+    lists = ListField(StringField())
+
+
+class Mixin(object):
+    number = IntField()
+    string = StringField(choices=(('One', '1'), ('Two', '2')))
+
+
+class Base(Document):
+    pass