Update grappelli/templates/admin/includes_grappelli/header.html to show custom user name

Notice that the name of the user is lost in the navigation bar:

![grappelli_no_username](https://f.cloud.github.com/assets/838679/55678/4726f686-5ae0-11e2-96b5-df0e5f356f8a.png)

If a programmer defines a custom user model (new in Django 1.5) with out first_name
or username fields 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_full_name 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.