Fixed #15284 - improved example jQuery code for adding X-CSRF-Token

Using the ajaxSend event is better than beforeSend, because the beforeSend
callback can have only one value, which makes it painful if it is needed by
multiple bits of javascript.

Thanks to LukeMaurer for report and initial patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15515 bcc190cf-cafb-0310-a4f2-bffc1f526a37