Improved DictFields

Allow searching multiple levels deep in DictFields
Allow DictField entries containing strings to use matching operators

Thanks again to @theojulien for the initial code #108

tests/queryset.py

@@ -1539,7 +1539,7 @@ class QuerySetTest(unittest.TestCase):
         t = Test(testdict={'f': 'Value'})
         t.save()
 
-        self.assertEqual(len(Test.objects(testdict__f__startswith='Val')), 0)
+        self.assertEqual(len(Test.objects(testdict__f__startswith='Val')), 1)
         self.assertEqual(len(Test.objects(testdict__f='Value')), 1)
         Test.drop_collection()