Adding *all* our StormpathUser model fields.

This commit adds in all the necessary model fields we need to represent our
StormpathUser in the local database.

If you're wondering why some of the field names don't match what's in the SDK --
it's because I'm trying to keep the user model consistent with the normal Django
User model -- this way developers won't be confused.
Adding StormpathUser.__str__().

This allows us to have pretty representation in the CLI.