Handle old named (referenced) docs

Refs #199

mongoengine/base.py

@@ -23,13 +23,20 @@ class ValidationError(Exception):
 _document_registry = {}
 
 def get_document(name):
-    if name not in _document_registry:
+    doc = _document_registry.get(name, None)
+    if not doc:
+        # Possible old style names
+        end = ".%s" % name
+        possible_match = [k for k in _document_registry.keys() if k.endswith(end)]
+        if len(possible_match) == 1:
+            doc = _document_registry.get(possible_match.pop(), None)
+    if not doc:
         raise NotRegistered("""
             `%s` has not been registered in the document registry.
             Importing the document class automatically registers it, has it
             been imported?
         """.strip() % name)
-    return _document_registry[name]
+    return doc
 
 
 class BaseField(object):