Moved the various Debug classes in django.template to a new module, debug.py,...

Moved the various Debug classes in django.template to a new module, debug.py, so they're only loaded if DEBUG=True. This led to a DEBUG=False memory savings of one 4-KB memory block on my machine, according to ps

git-svn-id: http://code.djangoproject.com/svn/django/trunk@6969 bcc190cf-cafb-0310-a4f2-bffc1f526a37