db_field and @property AttributeError

If you have a field on a Document that uses a db_field value that is the
same as a property on the Document itself, then an AttributeError is
raised when trying to fetch that instance from Mongo.

https://github.com/MongoEngine/mongoengine/issues/26

Fix taken from:
https://github.com/MongoEngine/mongoengine/commit/9e7ea64bd2cc4b42494b574750ff2afa5e8237b8
Merge pull request #3 from iainb/master

db_field and @property AttributeError