Update grappelli/templates/admin/includes_grappelli/header.html

If a programmer defines a custom user model (new in Django 1.5) with out first_name
or username then the user will not see its name or identification at the top right 
corner button (grp-user-options button).

with my change the template calls the  calls the new function get_short_name from
the customized user.

There are other options like get_username and get_fullname but i think get short
name is good for this case.

If the user don't define the method get_short_name in its custom user then the
name will be lost again.