Added missing is_superuser field

Without it, GaeUser.objects.create_user fails.
Merge branch 'andrei-fix-gae-abstract-user' of https://github.com/andreipetre/djangae into andreipetre-andrei-fix-gae-abstract-user
Improve the help text
Merge branch 'andreipetre-andrei-fix-gae-abstract-user'
Merge branch 'master' of https://github.com/potatolondon/djangae