Updated docs and added a NotRegistered exception

For handling GenericReferences that reference documents that haven't
been imported.

Closes #170

mongoengine/fields.py

@@ -339,7 +339,7 @@ class ListField(BaseField):
 
         if isinstance(self.field, ReferenceField):
             referenced_type = self.field.document_type
-            # Get value from document instance if available 
+            # Get value from document instance if available
             value_list = instance._data.get(self.name)
             if value_list:
                 deref_list = []
@@ -522,6 +522,9 @@ class GenericReferenceField(BaseField):
     """A reference to *any* :class:`~mongoengine.document.Document` subclass
     that will be automatically dereferenced on access (lazily).
 
+    note:  Any documents used as a generic reference must be registered in the
+    document registry.  Importing the model will automatically register it.
+
     .. versionadded:: 0.3
     """
 
@@ -648,7 +651,7 @@ class GridFSProxy(object):
         if not self.newfile:
             self.new_file()
             self.grid_id = self.newfile._id
-        self.newfile.writelines(lines) 
+        self.newfile.writelines(lines)
 
     def read(self, size=-1):
         try: