Fixed #6862 -- Refactored debug traceback extraction into an easy-to-use class.

Aside from being a little easier to read and use, this means you could subclass
a request/response handler class (from django.core.handlers) to add your own
traceback extraction handling in non-DEBUG environments and reuse this code.
Thanks, Ned Batchelder.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7927 bcc190cf-cafb-0310-a4f2-bffc1f526a37