Small refactoring of django.core.management to allow a custom argv to be...

Small refactoring of django.core.management to allow a custom argv to be passed into execute_manager().  This makes custom manage scripts with extra options possible

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3082 bcc190cf-cafb-0310-a4f2-bffc1f526a37