Customizable default email address.

Hi, first of all i have to say - you've made a very useful application.
The little problem i faced with using it is an empty emails  being created for lazy users. I have a email validation inside the pre_save signal handling for users. At this time there is no way to distinguish lazy user from the usual one due to the LazyUser instance is still not created. That's why i propose to use customizable email address for lazy user. It just allows such a distinction.