title: Exclude fields in Django admin for users other than superuser
body: <p>I have a simple <code>MyUser</code> class with <code>PermissionsMixin</code>. <code>user.is_superuser</code> equals <code>True</code> only for superusers. I'd like to be able to do something similar to this in my <code>admin.py</code>:</p>
  
  <pre><code>    if request.user.is_superuser:
          fieldsets = (
              (None, {'fields': ('email', 'password')}),
              ('Permissions', {'fields': ('is_admin','is_staff')}),
              ('Place', {'fields': ('place',)}),
              ('Important dates', {'fields': ('last_login',)}),
          )
      else:
          fieldsets = (
              (None, {'fields': ('email', 'password')}),
              #('Permissions', {'fields': ('is_admin','is_staff')}),
              ('Place', {'fields': ('place',)}),
              ('Important dates', {'fields': ('last_login',)}),
          )
  </code></pre>
  
  <p>Basically I want my users to be able to create other users, but not give them admin or stuff permissions. Only superusers should be able to do that.</p>