Added store_property to Fieldmanager_Datasource_User

I ran into a use case (replacing the default Co-Authors Plus user mapping meta box) where it made more sense to store user_login than ID. Therefore, I've added 'store_property' which allows you to save 'user_login', 'user_email', 'user_nicename' or 'ID' (the default).

Reciprocals don't work with anything other than ID so it throws an error if you try to enable that and the code validates it in appropriate places as well. This is going into production on the site that required it, so I'd like to get this into master. 
Good point, I think we should check that against a whitelist too.