Add get_user_document and improve mongo_auth module

  * Added a get_user_document() methot to access the actual Document class
    used for authentication.
  * Clarified the docstring on MongoUser to prevent its use when the user
    Document class should be used.
  * Removed the masking of exceptions when loading the user document class.
Merge pull request #423 from ncortot/get_user_document

Add get_user_document and improve mongo_auth module